H5rdmtoolbox

- 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

- fixed unnecessary call in `create_dataset`, which writes the data twice. Now, the time data is written is comparable
to the time `h5py` needs to write the data (for small datasets `h5py` is still faster due to the (constant)
overhead, `h5tbx` adds).

major changes:

- zenodo is not a dependency anymore but is implemented as a new subpackage of the toolbox
- zenodo is part or `repository` which is designed to provide interfaces to different data repositories (however,
only `zenodo` is implemented at the moment)
- the database architecture is changed similarly, such that it has a more logic structure
- both above changes follow a more or less strict inheritance structure from abstract classes defining the interface to
repositories or databases (databases are meant to be local, like mongoDB, SQL, etc., repositories are online data
storage, like zenodo, which allows searching for metadata but not within the raw files.)
- python 3.8 until 3.12 inclusive are supported
- IRI as persistent identifier is now supported, which fulfills the F3 requirement of the FAIR principles ("Metadata
clearly and explicitly include the identifier of the data they describe", https://www.go-fair.org/fair-principles/)
- package renaming and reorganization: `conventions` is now `convention`, `layout` is now a module, new is `repository`
- usage of IRI (persistent identifier) is now supported

- scale and offset is now implemented in the package is should not longer be defined in a convention.
- bugfix normalization extension
- bugfix exporting xr.DataArray built with the toolbox to netCDF
- support usage of IRI to describe metadata

- bugfix requirements
- add `$exist` `find()`-methods inside of HDF files
- bugfix 0D time data as dimension
- module query functions (`find`, ...) can guess the filename from objects that have
with `hasattr(hdf_filename, 'hdf_filename')`