Consider about adding back in Mercator projection

[thareUSGS]

The Mercator projection historically was a popular projection for planetary maps. There is some interest in re-listing Mercator as an option since it has been recently widely used in web maps. Original 2000 codes listed it as XXX74 (e.g., 49974), defaulting to center meridian = 0.

Perhaps wait until next IAU report (ETA -- soon-ish) to add this in -- even if an addition should be backwards compatible.

[AndrewAnnex]

@thareUSGS and @J-Christophe take a look at #5 as I'd like to not have to wait for the next IAU report, I've also made OSGeo/PROJ#3645 to patch things on PROJ

[AndrewAnnex]

#5 addresses this issue for this repo

[thareUSGS]

@AndrewAnnex for global maps (and planetary), I generally like to at least have clon=0 AND a clon=180 for those who want a more 0 to 360 data set. However, since this is really to fit into existing applications/services, do you ever see those being able to support clon=180. Basically should we add a code "91" to be clon=180?

[AndrewAnnex]

@thareUSGS I could add that but I think the code would have to be "95" to follow the standard used elsewhere here, a bigger question is if psuedomercator even allows for the idea of a anti-meridian centered mercator projection. I don't think many tools even respect the clon=180 for the equirectangular case as-is though so hopefully this isn't a blocker issue for you and this pr

[thareUSGS]

Good catch, yes if would be 95. I am tempted to leave it out for now since this is for matching existing services.