Recast `partition_mesh` in terms of generic part identifiers (#308)

* simplify partition_mesh interface

* add get_connected_partitions back to docs

* use opaque partition ID instead of number in partition_mesh

* use self/other instead of local/nonlocal

* fix compatibility

* remove unnecessary import

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

* eliminate fun

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

* stamp out remaining traces of fun

* rename membership_list_to_sets -> membership_list_to_map and store index sets as numpy arrays instead of python sets

* remove return_sets option from get_partition_by_pymetis

* fix bugs in MPIBoundaryCommSetupHelper

* flake8

* fix bug

* add a couple of fixmes

* handle groupless mesh case in dim/ambient_dim

* Revert "handle groupless mesh case in dim/ambient_dim"

not a good solution

* disable removal of empty mesh groups in partitioning

* fix some issues with partitioning docs

* remove empty-group filtering

* clarify part vs. partition terminology

* change some asserts to exceptions

* add a couple of FIXMEs

* cosmetic change

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* detect elements that belong to multiple parts

* add return_parts argument to partition_mesh

* add type hints to mesh partitioning

* Fix some annotations in mesh.processing

* add explicit part_id attribute to InterPartAdjacencyGroup

* cosmetic change

* fix some type annotations

* Dict -> Mapping

* fix outdated argument reference in docstring

* try using just dtype

dtype[Any] requires python 3.9

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