Dedicated backend class

Currently we don't have a dedicated class representing a backend (filesystem, hdf5, etcpp). It might be nice to have such a thing for various reason:
- Query backend specific information (default file ending, name, version, support for feature xyz)
- Tweak backend specific defaults (compression level, chunk size)
- specialized open method:
Currently the only way to open a file is via `File::open()`. If you now want to open a file with backend specific flags (compression mode and level) we could either add all these options (as generic property key-value pairs?) or we could have a specialized open method with all the backend specific options, i.e. `H5Backend::open("file.hd5", , )`.

Start by reviewing the existing File::open API and how filesystem and HDF5 backends are represented. The proposal needs a maintainer-defined interface and decisions about backend information, defaults, and specialized opening before completion can be judged.

Written by the indexing model from the issue text.