goesaodc_getStartTime(file) doesn't match other functions

[jonathancallahan]

All other goesaod_get~() functions accept an nc argument. This one is different and confusing.

We should change the name of this to something that communicates what it does more clearly:

goesaodc_convertFilenameToDatetime()

or

goesaodc_datetimeFromFilename()

or something else.