Added initial Matplotlib-based GUI slicer
Added simple save (but filename is always the same)
Implemented multi-point path, works without copying large datasets
Don't run callback functions if navigation bar buttons are pressed
Fixes to multi-point polygon path
Fix determination of extended polygon path, and allow polygons to be shown in GUI with 'y' keystroke
GUI fixes
Bring up Qt dialog for saving
Merge pull request #9 from astrofrog/gui-slicer

Added initial Matplotlib-based GUI slicer