[python] Add bindings for SubspaceBasis

This shares most of the code with the bindings for GlobalBasis. However, the constructor call is different and SubspaceBasis does not support basis.update(gridView).

Ingredients to achieve this:

• Let registerGlobalBasis() use the official interface to derive the Tree type and to construct a LocalView.
• Extract the generic part of registerGlobalBasis() as registerBasisType().
• Use registerBasisType() in the new registerSubspaceBasis() call.
• Add a python-method subspaceBasis(basis, ...). This does internally always use fully static tree paths which avoids the need for explicit bindings of HybridTreePath.
• Add missing functionality in Tree bindings.
• Make interpolation checks aware of subspace bases.