Add t0, t1, and t_mean time stamps to headers of gMap image cubes.

[scfleming]

This ticket is to add the same t0, t1, and t_mean values that would be output to a gAperture CSV light curve file to the corresponding gMap image cube, all other parameters being the same. E.g., if a user makes a gAperture light curve with time sampling 'x' and a gMap image cube with the same time sampling 'x', the values of t0, t1, and t_mean should be the same for each bin. Placing them in the FITS header of each extension will allow for glue mapping in the user interface we are developing, and generally help users correlate the two.

No strong preference for head keywords, could even use "t0", "t1", and "tmean" as the header keywords. In the comment section, make sure there is a brief desscription, e.g. for t0, "Start of the bin in GALEX time (seconds)."