Add Tobler-Mercator projection

[sphynx]

Resolves #584

[kbevers]

Thanks for adding this. I've requested a few changes in the docs otherwise this looks good.

[kbevers]

Thanks. I missed it before, we also need to add the version of inclusion: .. versionadded:: 6.0.0. Like this:

********************************************************************************
Tobler-Mercator
********************************************************************************

.. versionadded:: 6.0.0

Equal area cylindrical projection with the same latitudinal spacing as
Mercator projection.

[kbevers]

I am happy with this now. This should only be a 1 commit change. Do you want to squash the existing commits yourself (for full credit) or should just squash-merge it right away?

[sphynx]

I am happy with this now. This should only be a 1 commit change. Do you want to squash the existing commits yourself (for full credit) or should just squash-merge it right away?

Thank you for the review! Squashed.