Make obs directory match case of other datasets for ISMIP6 ocean forcing

[xylar]

#410 (comment)

@trhille:

Is it possible to make the 'Ocean_Forcing' part of this case-insensitive? It's a little awkward to have to have both an 'Ocean_Forcing' and an 'Ocean_forcing' directory when processing the obs and 2300 files, and it's not immediately apparent to the user why the ocean_thermal_obs case doesn't work when the ocean_thermal case does.

@hollyhan:

@trhille, this is a good point, and it would be ideal to have a case-insensitive directory for the ocean forcing files. The observational climatology file was originally only downloadable from the ISMIP6-2100 GHub endpoint, which is why I put the obs file in the same directory name Ocean_Forcing as that for the 2100 files. But the ISMIP6 committee has recently uploaded the obs data to the 2300 Ghub endpoint as well. I can see how to make the directory for the obs file case insensitive depending on which period_endyear is chosen in the config file in the following PRs I will create after merging this one.