Add ensemble functionality

[William-gregory]

This PR adds some functionalities needed for running ensemble experiments with SIS2.

The changes to SIS_get_input.F90 mean that the user can run a perturbed sea ice physics ensemble by passing unique SIS_override files to SIS_input_nml via:

parameter_filename = 'INPUT/SIS_input','INPUT/SIS_layout','INPUT/SIS_override_%E'

The changes to SIS_sum_output.F90 mean that a seaice.stats file will be created for each ensemble member, allowing the user to track conservation statistics for each member.

The changes to SIS_fixed_initialization.F90 mean that a sea_ice_geometry file will be created for each ensemble member. This was needed because the simulation crashes during initialization when running a large (30-member) perturbed sea ice physics ensemble from a cold start (input_filename = 'n'). It seemed like all the ensemble members were trying to read/write the same sea_ice_geometry file. This fix of having one geometry file per member is not the most elegant solution, but at least the model runs.

[theresa-cordero]

When using this with the Arc_11k simulation and 30 ensemble members using 258 nodes the run was crashing during the writing of the sea_ice_geometry file.

I think the solution Will suggested would work, but if it was possible to have just the first ensemble member write the sea_ice_geometry file that would be better.

[theresa-cordero]

Writing the sea_ice_geometry file can be turned off through on its own, which is the solution I have been using.

[William-gregory]

When using this with the Arc_11k simulation and 30 ensemble members using 258 nodes the run was crashing during the writing of the sea_ice_geometry file.

I think the solution Will suggested would work, but if it was possible to have just the first ensemble member write the sea_ice_geometry file that would be better.

Yes, good point! I just made this change, so now only ens_01 writes the geom file. Can confirm that this works in a 30-member ensemble job.