H5rdmtoolbox

- 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

- bugfix: Setting a default value for toolbox validators in convention yaml file was not working. Fixed it.

- simplified and clean up much code, especially convention sub package
- added identifier utils
- updated and improved documentation