Improve freeze/thaw usability (#218)

* Check for matching actxs in DOFArray arithmetic

* Clone _setup_actx to permit catching leaks

* Point arraycontext at freeze-thaw-usability in req.txt

* Fix simple-dg for missing thaws

* Add missing thaw in test_nodal_dg_interop

* Same-actx check in DOFArray: Use _cls_has_array_context_attr

* Point requirements.txt back to main for arraycontext