ROMS-only example

[TomNicholas]

Presumably it is possible to run ROMS without Marbl? This might be another nice example to include.

[dafyddstephenson]

Indeed - the roms_marbl_example repo has a no_cstar branch that actually contains three cases, NOBGC, BEC, and MARBL. C-Star currently just treats a single case with the MARBL version. I floated the idea of multiple example blueprints but it seemed to clutter things up a bit. No reason not to include them as manually constructed Cases in the docs though