Remove multipolygons / relations from OSM data extract during project creation

[spwoodcock]

Describe the bug

• Often we have a set of polygons in OSM that are linked together with a relation element.
• E.g. here we have a few adjacent polygons that in the real world represent an apartment building with multiple internal units that share walls, but all have separate entrances and no internal passages between units.
  
• Each Polygon is a separate geometry when extracted from raw-data-api, but the overarching relation (i.e. the entire apartment building) is represented as a MultiPolygon in the extract too.
• As a result we have overlapping geometries: this causes issues in ODK Collect when we try to select a geometry to map, as the MultiPolygon might overlay the individual polygons, meaning we can't select them.

Possible solutions

• Option 1: omit the multipolygons within the raw-data-api config. This is probably possible with some configuration in the YAML (preferred, as this would mean the flatgeobuf extract only contains what we need).
• Option 2: strip out the MultiPolygon geometries from the data extract prior to creating the ODK Entities, to prevent mapping the MultiPolygon relation.

Screenshots

The issue shown in ODK Collect:

Additional context

• This was a problem discovered during a mapping campaign.
• The relation information is useful in the general context of OSM, to link multiple geometries together. We just don't need it for mapping.
• I think the rendering order of the geom in ODK Collect is random, meaning we can't know if the MultiPolygon will be on top of the Polygons or not.