Add a command `oq nrml_from`

[micheles]

In particular, it must be able to convert a source model from .gpkg format to .nrml format (except for nonparametric sources). This is difficult in general (one must take into account duplicated source IDs, logic tree structure...) but easy if one consider one file at the time, so we would start from there. After this is done the commands oq to_shapefile and oq from_shapefile can be finally removed.