Misc. distributed fixes/tweaks (#208)

* tweak a couple of things in mesh partitioning

* add NotImplementedError for distributed meshes in as_python

doesn't currently check for InterPartitionAdjacencyGroup

* clean up array initializing

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* Narrow fagrp error check in mesh-to-python

* Make fagrp type check in mesh.as_python even more restrictive

Co-authored-by: Alex Fikl <alexfikl@gmail.com>
Co-authored-by: Andreas Klöckner <inform@tiker.net>