Add geometry_type to layer definition

[benstadin]

Some renderers (like ours) require to map a geometry type to a layer. Therefore I propose to have an optional field "geometry_type" for the layer definition. The geometry_type should relate to one of the simple feature types of defined in GeoJson [1]:

... geometry type refers to seven case-sensitive strings: "Point", "MultiPoint", "LineString", "MultiLineString", "Polygon", "MultiPolygon", and "GeometryCollection"

[1] https://tools.ietf.org/html/rfc7946#section-1.4