Add argument SHPC-dir to script `inst_eddies_Aviso.py`. We want to be
able to run `inst_eddies_Aviso` in parallel for several slices. Since
we write `used_config_nml.txt` to the current directory, we need to
run the parallel instances from different directories. So the current
directory cannot always be the directory containing the SHPC to be
appended to.