PostGIS Topology support Parallization for some functions for PostgreSQL 9.6

[robe]

There are some functions in topology that lend themselves to parallelization. Sadly since most are used for building topology, parallelization would probably never kick in since it doesn't for action queries.

Anyrate ones that come to mind that should be marked parallel safe

topoelementarray_append which supports topoelementarray_agg totopogeom asgml geometry cast function