Change MeshElementGroups from Record to dataclass (#283)

* make _VisConnectivityGroup a dataclass

* make _ConnectionBatchData a dataclass

* make MeshElementGroups dataclasses

* make NodalAdjacency and FacialAdjacencyGroup dataclasses

* clean up after dataclas conversions

* deprecate constructing MeshElementGroups using __init__

* remove most deprecated usage of MeshElementGroup

* add a little test for MeshElementGroup construction

* rearrange MeshElementGroup attributes in docs

* remove automethod

* Add deadline to MEG constructor deprecation

* Flag MEG make_group, constructor code duplication

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