Add dataset with ontological labels from CELLxGENE

[mxposed]

A lot of effort went into developing cell type ontologies and harmonization of labels in collections of single-cell datasets, such as CELLxGENE.

Some methods use the fruit of these efforts to learn directly on the ontological labels.

We should add a dataset with these labels to

• use harmonized labels across studies (this would allow to put a whole dataset into the test subset, potentially addressing Label projection != label transfer #1 )
• do justice to such methods

Additional context
See #19