Remove procedure `write_aux`. Write to `ishape_last.txt` in procedure
`write_snapshot` and write `grid_nml` in procedure `input_ssh`. In
`input_ssh`, if `grid_nml.txt` already exists, check consistency with
input NetCDF files. Downgrade dummy arguments `corner_deg` and
`step_deg` of `input_ssh` to local variables. Read and write
`grid_nml` at SHPC level, only one file for both orientations and all
slices.