[schema] allow for an optional subdirectory `prov/[prov-<label>]`

[bclenet]

1. Change the schema to allow for an optional subdirectory prov/[prov-<label>]
2. This also implies to add the optional subdirectory prov/[prov-<label>] in the auto-generated template for provenance files (macro MACROS___make_filename_template).