Generalize tensor product spaces (#41)

* add a generic tensor product function space

* extend tensor_product_nodes to non-homogeneous dimensions

* fix up tensor product basis

* add a TensorProductShape too

* add TensorProductShape to docs

* do not use math.prod

* simplify tensor product basis

* add some more types and docs

* fix tensor product gradient and add tests

* remove useless prints

* start modifying QN and Hypercube to inherit TensorProduct

* fix or silence some mypy errors

* fix remaining issues with tensor product spaces

* update docs to mention tensor products

* remove some unnecessary changes

* test and fix node tuples for tensor products

* extend submesh_for_shape to tensor product shapes

* Use ABC for Shape, FunctionSpace, Basis

* improve variable names

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

* attempt a cleaner _node_tuples_for_tp implementation

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

* fix variable renaming

* remove _quadrature_for_qn

* document order in space_for_shape

* remove _space_for_hypercube

* remove _random_nodes_for_hypercube

* add some docs for the internal _TensorProductBasis classes

* abstractproperty is deprecated

* reword docs a bit

* do not squeeze in random_nodes_for_shape

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