Fix/update the documentation for CALWF3 and associated standalone functions

[mdlpstsci]

The current on-line documentation for calwf3, as well as its components which can be run in standalone mode, is incorrect and out-of-date. The documentation and any associated diagrams need to be updated to reflect the true state of the pipeline software. This issue is related to Git PR#61. This issue is scheduled as JIRA HLA-759.

[mackjenn]

Currently, readthedocs lists UVIS steps in a bulleted list (below Fig 3), with 'Unit conversion' coming just before FLASHCORR, which is incorrect.

Below that is a section called 'Unit Conversion to Electrons', which states:
The UVIS image is multiplied by gain right after BIASCORR, converting it to ELECTRONS. This step is no longer embedded within FLATCORR.

This is also incorrect, since it's applied after FLATCORR. Your testing notes on this were in the discussion on the Saturation Maps issue 558

We also discovered that the reference files FLSHFILE, SINKFILE, DARKFILE are in units of either electrons or electrons/sec, and that calwf3 applies the gain to these files before correcting the data which is still in units of DN.

I'm tagging @starivera on this ticket for followup. Do we need to open a new ticket separate from the SATUFILE discussion to update readthedocs?

[mdlpstsci]

@mackjenn No. We can use this ticket and associated PR #68. Thank you for the reminder on these issues. I will schedule the doc update for the next sprint.