Save scenario-specific metadata in the output file

This allows scenario files to define metadata tables, whose contents
will be included in the simulation output files.

src/pypfilt/config.py

@@ -428,6 +428,11 @@ def make_params_fn(scen_data, settings: _Settings,
             'name': scen_name,
         }
 
+        # Save scenario-specific metadata, if any.
+        scen_meta = scen_data.get('metadata', {})
+        if scen_meta:
+            params['scenario']['metadata'] = scen_meta
+
         return params
 
     return params_fn