implemented FrameTime.to_excel; implemented numerical autorename in export to file methods (no test for second feature)
changed autorename format; fixed bug in autorename; wrote tests for autorename
added set_units; changed export tests to set units before attempting export
export methods auto-set units if None
removed extra data files that were messing up tests
export methods now return location of exported file
export methods auto datetime-stamp files, tests changed to reflect this
tests clean up better now
export functions no longer require a units arg; uses default. raises Error if default not specified
added a lot of error checking
fixed refactoring typo