Back-end: distinguish between creation and opening of entities

Currently the back end classes (such as BlockHDF5) have constructors used for both, creation and opening of entities.
It would be nicer to used constructors just for opening entities and create them using a static member function `create`.

This is also related to #305

Review the back-end classes mentioned in the issue, especially BlockHDF5, and read the discussion in issue #305 first. Map which constructors currently create or open entities; completion means the intended creation and opening paths are consistently separated across the back end.

Written by the indexing model from the issue text.