Automatically add comments describing each map plane.

[ossama-othman]

MaRC has enough information to automatically add COMMENT fields in the map FITS header that describes each map plane, such as type of plane, plane data scaling/offset factors, etc. Similar things can be done with the map grid extension XCOMMENT fields, including latitude/longitude grid line intervals, bounds, etc. Generate such COMMENT and XCOMMENT fields automatically.

[ossama-othman]

This is a work-in-progress. The changes from the automatic data scaling pull request (#37) introduced code that writes the scale and offset needed to obtain the true values of the MaRC virtual images, e.g. Mu and latitude images, to FITS HISTORY entries in the map FITS file.