Make translator between grid subset names and indices

[eldond]

Could be hard to remember what subset number you need if you want to see outer midplane. Maybe functions that take subset index as an argument could also take a string and translate it using a translation table.

• Example: in src/subset_tools.jl : get_grid_subset_with_index() takes grid_subset_index (an integer) as an input. An alternative to this function could accept the subset name instead and look up the index.
• Anything that accepts grid_subset_index or subset_index or something equivalent could work the same way.
• A function could be made for translating between index and string.

[anchal-physics]

Addressed in #37, ProjectTorreyPines/SOLPS2imas.jl#31, ProjectTorreyPines/FusionSyntheticDiagnostics.jl#24, and ProjectTorreyPines/SOLPS2ctrl.jl#48

Now the function is called get_grid_subset() and it works with both an integer for index or string for a name.

Kudos @dautt-silva for completing first set of PRs!