Added documentation for three oblique mercator projections

[eemcmullan]

Description of proposed changes

Added three files to examples/projections/cyl for the three different oblique mercator projections. Included a brief definition of oblique mercator projections, descriptions for each option, as well as working code examples. This is in response to issue #1073. Please let me know if something needs to be fixed; I would be more than happy to change it!

Fixes#1073

Reminders

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• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

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[michaelgrund]

Thanks for opening this PR @eemcmullan. Unfortunately it seems like @willschlitzer started to work on this topic this morning in #1250. Maybe you can bundle up your efforts?

[eemcmullan]

@michaelgrund Ahh I see that now. Of course, I can reach out to him about bundling!

[maxrjones]

Thanks for submitting this pull request! Could you please add links to these projection examples in the projection table as part of this pull request? The source text for the projection table is in examples/projections/table/README.txt. There are three lines (L56, L58, and L60) that need to be updated, for example:

 - Oblique Mercator, 1: origin and azim

will link to the first example if it is changed to this:

 -:doc:`Oblique Mercator, 1: origin and azim </projections/cyl/cyl_oblique_mercator_1>`

[seisman]

The PR looks good to me. Thanks, @eemcmullan.

@michaelgrund@willschlitzer Please give it approval if you're OK with the PR.

[willschlitzer]

Looks good! Thanks for this addition, @eemcmullan!

[eemcmullan]

I was happy to do so! Thanks all for your help! @willschlitzer@seisman@michaelgrund @meghanrjones

[seisman]

@meghanrjones Please give it a final review and feel free to merge it with your admin permission, since the original issue #1073 was opened by you.

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