Add flatten and unflatten to numpy (#196)

* add flatten and unflatten to numpy

* use flatten_to_numpy in tests

* fix thawing in flatten_to_numpy

* handle a bare ndarray in unflatten_from_numpy

* remove discr is None in unflatten_from_numpy

* add a strict flag to flatten and unflatten

* use better key in flatten/unflatten memoized kernels

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

* add some commas to docs

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

* Clarify docs for unflatten in the presence of multiple DOFArrays

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