Fix/plex distribute

Fixes parallel mesh distribution for builtin meshes created via DMPlexCreateFromCellList.  Also removes ability to generate gmsh meshes unless we have gmshpy installed (the invocation of the gmsh binary has been incredibly flaky).

I think this also excises the last in-codebase use of gmsh2triangle (and a reference to fldecomp).  But I haven't binned that yet, awaiting Michael's gmsh reader changes.