add METIS_PartMeshDual to part_mesh (#46)

* add METIS enum; enhance part_mesh; add tests

* specify all `part_mesh` arguments in test/*.py

* resolve comments

* import pymetis.metis

* update doc

* fix a grammar error in the doc

* fix linter errors: attempt#0

* remove submodule metis; revise gtype doc; add error msg to wrap_part_mesh; TODO: doc enum

* update doc of enums

* update doc of enums

* fixed a linter error

* Use references to refer to graph types

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