Support updating `.vmr` files with new gases

[joshua-laughner]

GGG users may want to add new gases to their retrievals, either for lab work or experimental atmospheric retrievals. This is doable for those who use ginput locally, as they can update their climatological .vmr file and rerun the vmr subcommand. However, since most users get their .mod and .vmr files from the central automation, they currently cannot easily generate new .vmr files.

A compromise solution is a new subcommand, perhaps update-vmrs, that takes a new climatological .vmr file and modifies one or more existing .vmr files match the climatological one. I could see several operational modes:

• add would only add new gases.
• replace would replace any gases in the current .vmr files with new profiles derived from the climatological file. This would probably need a flag to indicate whether or not to replace the "special" gases that don't normally come from the climatological file.
• match would act like replace and, in addition, remove any gases from the current .vmr files not in the climatological one.

This would use the "classic" gsetup approach to vary gases based on the tropopause height, latitude, time of year, etc., so users must include the secular trend, latitude gradient, and seasonal cycle coefficients in their header along with the tropopause height, date, and latitude that the profile represents.