HDF5 interface: add write_dict_to_hdf() and read_dict_from_hdf()

[jan-janssen]

This pull request refactors the helper functions in the pyiron_base.storage module and adds the write_dict_to_hdf() and read_dict_from_hdf() functions. The write_dict_to_hdf() function takes a dictionary where the keys represent the path in the HDF5 file and the values represent the objects which should be stored in these nodes. The corresponding HDF5 groups are created automatically. The read_dict_from_hdf() function reads the nodes from a given HDF5 group and subgroups can be included using the optional group_paths parameter. The function returns a hierarchical dictionary which can be directly converted to a DataContainer.

[jan-janssen]

@pmrv As discussed in the pyiron meeting, I plan to go ahead and merge this so we can focus on defining the interface in a separate pull request.

[jan-janssen]

@pmrv I fixed the remarks and would like to merge this. So if you have time to take another look that would be great.

[jan-janssen]

While I made a couple more changes to add the read_dict_from_hdf() function - still the pull request is now again ready for review.

[pmrv]

I left a number of small remarks, but can be merged after those are considered without re-review.

The only bigger thing I've seen is the slash parameter. While it was present before this PR introduces a lot of code now that offers it, but afaik we never use this, so it can be just removed completely imo

[jan-janssen]

The only bigger thing I've seen is the slash parameter. While it was present before this PR introduces a lot of code now that offers it, but afaik we never use this, so it can be just removed completely imo

I would like to make the slash parameter as well as the compression setting accessible to the outside once we release the stand-alone HDF5 interface, so I already added it to the central functions in the helper class.