[proposal] adding pm/prime_meridian as an option for Globe?

[snowman2]

The PROJ parameter pm can support a string or a number.

The prime meridian is used when defining a datum or for handing the -180/180 issues.

• https://pyproj4.github.io/pyproj/stable/build_crs.html
• https://lists.osgeo.org/pipermail/proj/2020-April/009540.html

So, I propose adding prime_meridian as an input parameter to the Globe object that maps to the pm PROJ parameter.

This would enable:

    from cartopy.crs import CRS as cCRS, Globe
    from pyproj.crs import CRS

    proj_crs = CRS.from_epsg(4326)
    globe = Globe(
        ellipse=None,
        semimajor_axis=proj_crs.ellipsoid.semi_major_metre,
        semiminor_axis=proj_crs.ellipsoid.semi_minor_metre,
        inverse_flattening=proj_crs.ellipsoid.inverse_flattening,
        prime_meridian=proj_crs.prime_meridian.longitude,
    )
    proj_dict = proj_crs.to_dict()
    cart_crs = cCRS(proj_dict, globe=globe)

[snowman2]

Not as important with #1808