DOFArray: Pickle/unpickle tags (#318)

* DOFArray: Pickle/unpickle tags

* make unpickling backwards compatible

* use dict instead

* add test

* fix axes tags

* make sure to always save tags

* fix axes_tags

* axes_tags fixes and various asserts

* support empty DOFArray

* Simplify tag storing

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* call freeze() for metadata inference

* add warning for unpickling backwards mode

* add link to comment

* test with Pytato actx too

* skip test_flatten_unflatten with pytato actx

Co-authored-by: Alex Fikl <alexfikl@gmail.com>