Replace output struct by writing to nc file

[JanJereczek]

Until now, the output of FastIsostasy was relatively sparse in time, as for instance in the case of ICE6G (128 time steps). Now that we are working with denser sampling in time for future scenarios, it appears that a big memory limitation is the fact that we construct Output to store the results. This should be replaced by writing current states to a NetCDF file. The API for that should look roughly like:

opts = (output_netcdf = "myfile.nc")
fip = FastIsoProblem(..., opts = opts)
solve!(fip)

The default value output_netcdf = nothing should skip the saving procedure.