code: def make_catalog_sources(catalog_roi_model, source_names):
      
      sources = {}
      for source_name in source_names:
          sources[source_name] = catalog_roi_model[source_name]
      return sources
docs: Construct and return dictionary of sources that are a subset of sources
      in catalog_roi_model.
  
      Parameters
      ----------
  
      catalog_roi_model : dict or `fermipy.roi_model.ROIModel`
          Input set of sources
  
      source_names : list
          Names of sourcs to extract
  
      Returns dict mapping source_name to `fermipy.roi_model.Source` object