Review complexity metric checking

[pp-mo]

I seem to have an ongoing problem with the flake complexity checking (McCabe).
It is perfectly true that many of the routines are rather over-complex, but the check only recently started failing.

This is simply disabled for now == line removed from setup.cfg, in #26

[pp-mo]

To fix this properly, various sub-tasks of (e.g. and especially?) check.Checker.check_mesh_var should probably be moved to subsidiary methods.
But that in itself is going to make the flow + logic hard to read, since there is already a fairly logical division into separate checking routines. So it needs some careful thought.