[treePath] enforce the index type of HybridTreePath

Internally HybridTreePath is expected use std::size_t or index_constant<k>. We now check that an explicit construction HybridTreePath adheres to this assumption and the helper functions treePath and hybridTreePath internally cast all arguments accordingly.

If HybridTreePath is instatiated explicitly, the template arguments might still map to other index types. We thus check these types and potentially trigger a deprecation warning.

fixes #13 (closed)