test_set_interior: document the new test!

[mnlevy1981]

The documentation should highlight the fact that this is an example of the full process necessary to bring MARBL into a new GCM (just wrap the interior_tendency_compute() and surface_flux_compute() calls in a time-stepping loop). It clearly shows what needs to be initialized, and how to determine what forcing is necessary.

[mnlevy1981]

Definitely need to update usr-guide/GCM-interface/surface_flux.rst and usr-guide/GCM-interface/interior_tend.rst to use examples from the new test rather than POP. Also need to look for things like usr-guide/GCM-interface/init.rst (which refers to *.input files that have been renamed to *.settings in this PR). At first glance, I don't think there is any new documentation to write, though.

[mnlevy1981]

Cleaned up init.rst in 5c442b9

[mnlevy1981]

Last few tasks are laid out in a MARBL meeting agenda

[mnlevy1981]

Also, we discussed cleaning up the marbl_io_read calls. So rough to-do list:

• [x] Call marbl_io_read() into local arrays, then copy those local arrays into marbl_instance
• [x] Write up unit testing
• [x] Write up regression testing
• [ ] ~Write up POP's implementation of saved state~

[mnlevy1981]

Per #25 going to wait for later issue to address saved state documentation

[mnlevy1981]

done as-of 6d1b272