Update more keywords to be weighted when making combo

[abhimat]

When making the combo image, the values for only the MJD-OBS and the DATE keywords in the final combo image file are updated with the weights. Additional keywords should also be updated to reflect the final weighted value.

Currently, almost all keywords' values in the combo file header are coming from the combo epoch's reference image, which is just the lowest FWHM frame going into the combo. This reference file is what is used for the base when drizzling and all FITS header keywords are just kept from the reference image file.

The following keywords should be updated with the weights:

• EXPSTART and EXPSTOP
• PMSANGL (and other pupil keywords?)
• Rotator angle keywords (e.g., ROTPOSN)?

I can make this change; making it into an issue so we can track it and possibly discuss any other keywords that may make sense to apply the weights to when we're saving out the final combo image. Weighted exposure time and pupil keywords will be helpful for applying AIROPA variable-PSF mode (as suggested by @skterry)!