UPDATE save => toFile to keep consistent naming scheme.
ADDED _hasData and has___Data functions for testing if objects contain data of given kind
REFACTORED FRET.plot() using subfunctions to improve abstraction and usability. Now plots donor/acceptor, fret, and/or FEC/FDC curves as a single figure with subplots using _subplot().
UPDATED to reflect improvements to FRET.plot()
ADDED isInt() to check if string is an integer; ADDED generators grouper() and groupat() which help chunk/batch a list by number (grouper) or conditional (groupat)