Evoke collection using a different meta data schema

[DunklesArchipel]

We currently support multiple metadata schemas in the metadata descriptor of the resource.

We could allow loading a different schema when the collection is created.

db = Collection(packages=..., meta_data_schem='echemdb')

If no schema is provided, we could simply use metadata as a descriptor.

In addition, we could implement a method get_metadata, which allows accessing different kinds of metadata in the metadata descriptor, which would otherwise require

metadata = [metadata for metadata in entry.package.get_resource('echemdb').custom["metadata"] if 'mds_name'  in metadata][0]

# replace by
metadata = entry.get_metadata('mds_name')