Red Cell button for GUI

[IanMGriff]

Added keyboard shortcut to change from red cell to not red cell by pressing C in the GUI. Added function flip_redcell to masks.py which implements the change, which is called within keyPressEvent in gui2p.py. flip_redcell updates the GUI to change the selected cell to red, and updates redcell.npy accordingly. Pressing C again on a cell marked as red flips the color, "deselecting" the cell, and again updates redcell.npy.