Add exposure duration (time), FWHM, and individual detection MJDs

[moeyensj]

Add exposure duration or time to frames, and add FWHM if available as well.

Each individual observation should support having any observation time within the range defined by the start of exposure + exposure duration. This is particularly important for the LSST use case where each individual DIASource from a single visit will have a distinct observation time (this is to account for the motion of the shutter as it opens and closes -- pretty cool!).