Consistency in input file names and input field names

[sit23]

Description

Isca uses many input files, most of them netcdf. In netcdf files each variable has a name, and so when Isca reads from an input file it needs to know both what the input file is called, and also what the input variable or field name is. For most input file cases (e.g. SST and sea ice in mixed_layer.F90) Isca is set up to assume that the file name and field name are the same:
E.g. for sst:
https://github.com/ExeClim/Isca/blob/master/src/atmos_spectral/driver/solo/mixed_layer.F90#L310
and
https://github.com/ExeClim/Isca/blob/master/src/atmos_spectral/driver/solo/mixed_layer.F90#L334

But for some variables, the field name can be specified separately, e.g. qfluxes here:
https://github.com/ExeClim/Isca/blob/master/src/atmos_spectral/driver/solo/mixed_layer.F90#L120

This lack of consistency makes Isca awkward to use with input files.

In addition, some input file names require the folder INPUT to be added before the file name, and some require .nc at the end, where others do not.

Recommendation:
All of Isca's netcdf input file options should be made consistent in terms of:

• Whether or not the folder is specified
• Whether .nc is required at the end
• That the field name and file name can be specified separately