H5rdmtoolbox

- calling the RDF accessor on an attribute name will only work if the attribute already exists. If not, an error is
raised.
- Likewise, if an attribute is deleted, the entry in the RDF accessor dictionary is deleted

- minor fixes
- add $in operator to query functions
- add argument `rebase` to layout specifications

- important changes:
- improved and consequent support of RDF/JSON-LD. This means, an HDF5 can be created from a JSON-LD file and vice
versa. The JSON-LD file
contains the structural and contextual metadata of the HDF5 file.
- namespaces are outsourced to `ontolutils`
- minor changes:
- When a file is opened with a filename which does not exist and mode is None, the file will NOT be created. This
was
the case in the past, but this may lead to unwanted empty files.
- Bugfix namespace creation
- some method renaming and refactoring
- accessors are refactored and improved (especially shifted away from xarray and fully integrated in hdf)

- Hotfix dumping json-ld data (dimension scales were the issue)

- Add codemeta namespace
- Improved json-ld export
- Updated qudt namespace
- colab notebook will be managed on a separate branch. the readme link points to the branch

- Improved assignment of IRI to attributes
- Export of a JSON-LD file possible
- Updated documentation
- bugfixes