def get_coordinate_conversion_session_for_type(self, source_coordinate_type=None, target_coordinate_type=None, proxy=None):
Gets an ``OsidSession`` associated with the coordinate conversion service and the given coordinate types.

        arg:    source_coordinate_type (osid.type.Type): the type of the
                source coordinate
        arg:    target_coordinate_type (osid.type.Type): the type of the
                target coordinate
        arg:    proxy (osid.proxy.Proxy): a proxy
        return: (osid.locale.CoordinateConversionSession) - a
                ``CoordinateConversionSession``
        raise:  NullArgument - ``source_coordinate_type,``
                ``target_coordinate_type,`` or ``proxy`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_coordinate_conversion()`` or
                ``supports_visible_federation()`` is ``false``
        raise:  Unsupported - ``supports_coordinate_record_types_for_con
                version(source_coordinate_type ,
                target_coordinate_type)`` is ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_coordinate_conversion()`` and
        ``supports_visible_federation()`` are ``true``.*