`geojson` output should create MultiPoint/MultiLineString for homogeneous collections

[clarisma]

See https://www.rfc-editor.org/rfc/rfc7946#section-3.1.8:

Furthermore, GeometryCollections composed of a
single part or a number of parts of a single type SHOULD be avoided
when that single part or a single object of multipart type
(MultiPoint, MultiLineString, or MultiPolygon) could be used instead.