- add accessors: geometry_n, interior_ring_n
- add num_geometries, num_rings, num_interior_rings
- add nodes_to_points for converting every node in a geometry
to a multipoint geometry
- add segments_to_lines for converting every segment in a geometry
to a multiline geometry
nodes_to_points and segments_to_lines are intended for use with
geometry generator symbology, eg to allow use of m and z values
for nodes/lines with data defined symbology.