Move some properties from Dataset to Datacard?

[hagenw]

In #13 we introduce the new audbcards.Datacard class that handles the rendering of the actual data card. We still continue with audbcards.Dataset as this might be handy to gather statistical information about datasets.

Having this in mind it might be better to move all the functionality that creates figures or audio player elements from audbcards.Dataset to audbcards.Datacard and focus on the actual values in audbcards.Dataset.