add polymer dataset

[jannisborn]

Add a dataset for polymers that creates an arbitrary tuple of smiles datasets.

• same params as smiles dataset but takes lists for each param, length of list corresponds to amount of smiles dataset.
• Single smiles language object for all of them.
• We need modification in smiles language to handle start and stop of individual smiles entities like etc (--> polymer_smiles_language class?)
• loader returns a tuple, one smiles per dataset

[drugilsberg]

Closed by #17