Clean up GWspatialParameters implementation

[ccarouge]

This is to be done once all the GW code is merged in main. Blocked until the merge is done. This allows for easier testing.

Most comments are recorded in #224.

We also need to deal with aligning with the coding standards (for the added code).

[rkutteh]

@ccarouge thanks for all the comments. Some I suspect will be resolved "automatically", once I am finished pushing all the remaining changes to src/offline/cable_parameters.F90 . The remaining I agree we can address after testing.