Handle metadata selection in a proper way

[Ghadjeres]

It is possible to choose which metadata to use when creating the dataset for the first time.

# fixed set of metadatas to use when CREATING the dataset
# metadatas = [FermataMetadatas(), KeyMetadatas(window_size=1), TickMetadatas(SUBDIVISION), ModeMetadatas()]
 metadatas = [TickMetadatas(SUBDIVISION), FermataMetadatas(), KeyMetadatas(window_size=1)]

Choosing which metadata to use can allow a faster creation of the dataset (it's actually the Key and Mode metadata which are longer to compute) and different generations.