ENH: Add LTA in/out to Tkregister2

[effigies]

Changes proposed in this pull request

• Add LTA input/output with inversion
• Reorganize input/output transforms to make future work on this interface a little easier
• Add an xor to fstal that follows this section of the help:

  --fstal

  Check and edit the talairach registration that was created during
  the FreeSurfer reconstruction. Sets the movable volume to be 
  $FREESURFER_HOME/average/mni305.cor.mgz and sets the registration file to be
  $SUBJECTS_DIR/subjectid/transforms/talairach.xfm. User must have
  write permission to this file. Do not specify --reg with this
  flag. It is ok to specify --regheader with this flag. The format
  of the anatomical is automatically detected as mgz or COR. By default,
  the target c_ras is temporarily set to 0 to assure that the target
  is properly centered. This is taken into account when computing 
  and writing the output xfm. To turn this off, add --no-zero-cras.

This is not a complete fleshing out of tkregister2.