Add more spatial extras (length, area, distance_to_filter and intersection_area)

[LionelZoubritzky-IGN]

This PR introduces a number of small refactors (each in their own commit) and finally adds four new spatial_extras targets: length, area, distance_to_filter and intersection_area. The latter two can only be used when there is a spatial filter.

Requests error when they are absurd, for example asking for area on a geometry which is stored as format: "geometry-point" in the schema. When they are not absurd but still impossible in practice (the same but with "geometry-any" that actually returns a Point for example), the required field is set to null.

Close #136, although we may want to add even more spatial_extra in the future. For the time being, these new spatial_extras allow answering new questions like:

• how long is Saône river exactly ? (length) -> 515,08 km
• give me the 5 largest parks in Tours (area) -> La Cousinerie (24ha), Sainte-Radegonde (16ha), ...
• what is the swimming pool closest to the "Lux" cinema in Caen? (distance_to_filter) -> Piscine du Sivom des Trois Vallées de Mondeville (1.37 km)
• what is the surface of forest within 5km of Orléans? (intersection_area) -> 1 039,3 ha