Save the scenario configuration in the output file

This saves a normalised version of the TOML configuration in each
simulation output file.

src/pypfilt/config.py

@@ -423,9 +423,11 @@ def make_params_fn(scen_data, settings: _Settings,
 
         # Record the scenario ID and name in the parameters dictionary, so
         # that it is recorded in each output file.
+        # Also record the (normalised) scenario configuration dictionary.
         params['scenario'] = {
             'id': scen_id,
             'name': scen_name,
+            'config_str': toml.dumps(scen_data),
         }
 
         # Save scenario-specific metadata, if any.