Loaders and constant drug_name and cell_line_name

[PascalIversen]

Overall, we will identify cell lines by "cell_line_name" and drugs by "pubchem_id". This is how we will structure the datasets now.
Additional changes:

• renamed all fingerprint files to "pubchem_id_to_demorgan_{bits}_map.csv"
• also have the same structure now for Toy_Data as everywhere else, but there, we just have the pubchem_id_to_demorgan_128_map.csv file