Provide user access to dataset dictionary in audbcards.Datacard

[hagenw]

Currently, the RST page of an data card is rendered by audbcards.Datacard._render_template(), which calls content = template.render(dataset), where dataset is a dictionary in the form of {'name': 'database-name', ...}, whereby the keys are used in the templates to specify what should be displayed on the RST page.

If a user would have access to dataset, e.g. in the form of audbcards.Datacard.dataset it would be very easy to extend a datacard by some special entries without the need to first update the audbcards package.
I also see two challenges we need to solve when adding this feature:

• We cannot just add it during __init__() as the dictionary is only filled when _render_template() is called
• The name dataset might not be the best choice as it is easy to be confused with an actual audbcards.Dataset object