docs: def define_crs(self, type, name=None, link=None, link_type=None):
          """
          Defines the coordinate reference system for the geojson file.
          For link crs, only online urls are currenlty supported
          (no auxilliary crs files).
  
          Parameters:
  
          - **type**: The type of crs, either "name" or "link".
          - **name** (optional): The crs name as an OGC formatted crs string (eg "urn:ogc:def:crs:..."), required if type is "name"
          - **link**: The crs online url link address, required if type is "link".
          - **link_type**: The type of crs link, optional if type is "link". 
          """
          if not type in ("name","link"): raise Exception("type must be either 'name' or 'link'")
          crs = self._data["crs"] = {"type":type, "properties":{} }
          if type == "name":
              if not name: raise Exception("name argument must be given")
              crs["properties"]["name"] = name
          elif type == "link":
              if not link: raise Exception("link argument must be given")
              crs["properties"]["href"] = link
              if link_type:
                  crs["properties"]["type"] = link_type