[get_hydrometrie_observations_tr] add an option for handling duplicate entries

The parameter code_entity is ambivalent and refers both to code_site and code_station (See https://github.com/BRGM/hubeau/issues/73).

Add an option 'entities' for filtering the API response:

• "station": keep only lines where code_station is not null
• "site": keep only lines where code_station is null
• "both": keep all lines related to stations and sites