Incorporate code like 'create_mesh' into pmesh

[ole]

We have a prototype a code called create_mesh_from_regions which creates a mesh from polygons. It simplifies the scripted mesh generation greatly and could be evolved to be part of pmesh. It is used by the Karratha study.

See ​https://datamining.anu.edu.au/anuga/inundation/pmesh/create_mesh.py and ​https://datamining.anu.edu.au/anuga/production/karratha_2005

Please have a think about what could be done along those lines