TODO

- Change control surface wedge cutout to two supported lines.

- Add support for sheeted trailing edge (bottom sheet/square stock.)

- Need a way to specify rib material thickness.

- Design is dirty after initial load from menu->open and shouldn't be.

- Support flap/aileron starting/stopped at the same station.

- Test/support full wing sheeting.

- Add an about me page/dialog?

- Sheeting needs to allow various positioning like other objects to handle
  tapered wings!

- (why?) wing: made a (bad) stringer shape with only 2 points!
  picking up inner rib of mirror side?  Maybe assume this is a 'normal'
  error and don't report ... or figure out how to distinguish these by more
  than station?  because both wings could share the central station.

- Dihedral should be summed with previous panel if wing is linked.

- Need to (a) stretch inner rib to account for dihedral and (b) cant it in
  the 3d model so it looks right.  Probably not a big deal for building
  but shows up annoyingly in the 3d model.

- Way to specify absolute position of assemblies.

- 1/14/2014: User bug report with mm units: leads to addition of fields to
  specify cut sheet and plan sheet dimensions, as well as working through
  the process of doing everything in mm.  Still an open issue with material
  width when not using "in" units.

- 1/1/2014: Relative positioning of sub assemblies.  In the current
  implementation this is accomplished by "linking" a wing to another wing.

- 12/27/2013: Add internal version knowledge, remote check for new version
  (in a task.)  Related: heads up message if we are loading a design written
  by a different version of creator.

- 12/27/2013: Control surfaces via gui.  Need to figure out a sane way to
  specify trailing edge of wing vs. control surface.  builder will split up
  trailing edge at flap boundaries and assign part = wing | flap accordingly.

- 12/26/2013: Control surface ... specifying position with anything other
  than absolute position seems to result in edge stringer misplacement.
  Fixed by adding an "xoffset" to a CutPos which is added after the position
  is evaluated (could be a percent of chord for instance which we don't know
  until the rib is made.)

- 12/22/2013: Fix output location of .svg files

- 12/22/2013: Check that .ac exists and is newer than .mad file before
  allowing view3d

- 12/22/2013: Beautify saved xml files (.mad) by using lxml instead of xml
  which supports the pretty_print=True flag when calling write()

- 12/21/2013: Spars are vertically aligned after twist rotation, stringers
  are always flush/tangent to the surface.

- 12/19/2013: When opening a new design, old wings aren't deleted.

- 12/19/2013: Warn if trying to build a dirty design (build is from the saved
  file, not in memory

- 12/18/2013: Curved taper via gui / spline curve.  The interface is a bit
  clunky, but see the yet to be written docs and examples.

- 12/18/2013: Tapered wing, bug with bottom sheeting 3d, predict num
  surfaces (?)  This was an edge case bug where I was counting number of
  vertices not number of surfaces.

- 12/18/2013: Add separate root/tip chord support for tapered wings.

- 12/17/2013: Airfoil selection via file/dialog box rather than typing the name.
  allow either / or via the new 'assist me' menu.

- 12/15/2013: Implement 'new' button that clears the current design andi
  starts from scratch.

- 12/15/2013: Catch deletion of structures as a design change so we can warn
  of unsaved changes.

- 12/15/2013: Implement change tracking so we can warn of unsaved changes
  if we are quiting, opening, or new-ing

- Fixed: 12/5/2013 (insert a point right at the cut so we can get a perfect fit)
  Trailing edge blending not happening now so there is a discontinuity with
  rib -> stock flow.  We should have support for this in code?
  Seems to work with symmetrical airfoils, but not asymetric airfoils?
  It's because we cheet the original contour points before cutout so we
  can't have the tail ends cross so there's a bit of a discontinuity from the
  last point to the tip ... in a low res build this is more pronounced.
  The fix I think would be to insert a contour point right at the trailing
  edge cut.

- Fixed: 12/4/2013
  builder can now set # of airfoil resample points and # of points used
  to build circles to control build speed vs. quality

- Fixed: 12/4/2013
  Just noticed a glitch in polygon winding on bottom sheeting, fixed.

- Fixed: 12/4/2013
  need to fix surf # estimation in sheeting when blending shapes and changing
  # of points from one rib to the next.

- Fixed: 12/4/2013 (pull extrusion from same set of points used to create
  3d model)
  trailing edge not correctly drawn on plans.
  leading edge has same problem I think?

- Fixed: 12/4/2013
  Need to adjust 'saved_bounds()' when part is rotated so post rotation parts
  get placed correctly and not relative to the original bounds before rotation.
  shows up more in high twist design.

- Fixed: 12/3/2013 (float/int implicit conversion bug, otherwise pretty much
  worked as intended!)
  test 3d sheet building when airfoil interpolates from one size/shape to
  another over the length of the wing.

- Fixed: 12/3/2013 (need to rotate/transform the seed points not just the
  cut polygon)
  leading edge stock doesn't follow "twist"
  trailing edge stock doesn't follow "twist"
  sheeting doesn't follow twist
  (I believe the problem is because these items are cutout before the twist
  is applied.)

- Fixed: 12/3/2013 (reimplmented algorithm with new approach)
  Sheet 3d model creation now far more robust and can handle basic wing
  sweep.

- FIXED: 12/2/2013 (python indent bug)
  Spars/stringers can get goofy, drop to bottom for one spar in some case
  with twist?