Add edit command

[griffin28]

Procedure: Graphically edit a curve on the screen. This command takes a curve to be edited. A copy of the curve is made and drawn as a scatter plot. By holding down the left mouse button the user can select points for removal. Points can be toggled back by holding down the shift key and the left mouse button. When the all points are selected, pressing Enter draws the new curve, pressing ESC cancels changes.
Usage: edit curve