WIP - Made lens auto-center display. Simplified front-end lens
interaction by adding x and y display_offset varabiales to lens
WIP - "works" with the graphical front-end now. Formulas for
calculating offsets in the lens aren't quite right
WIP - Added calculation of scale for lens. Switched sign of offset
Fixed typo of break in lens_move
WIP - Fixed logic in lens_move
WIP - Removed board argument to lens_move functions. Moved x,y offset
validation to lens_set. stubbed out graphical-ui controls
Added controls to graphical front-end. added arrow keys to both
Added comments/docstrings to lens
Added lens movement instructions to usage string/README