[selectors, facet] make is_instance and other EClassSelector subclass gather entity classes from 'select' and 'filtered_variable' context information. Closes #1796732 This allow for instance to use the filter form on rqlst where the filtered variable is not selected

if schema['TZDatetime'].eid is None:
    add_entity_type('TZDatetime')
if schema['TZTime'].eid is None:
    add_entity_type('TZTime')