TargetManager does not check if residues and targets have same length (for residue_to_class)

[SebieF]

Currently, the TargetManager only reads the sequences and labels, so we have a number of residues and a number of labels in the end. If their count does not match (e.g. SEQSEQ, 11101), there is no error thrown in the TargetManager class. The error is then thrown of course in the loss calculation, where prediction and y dimensions have to match.

Of course one could argue that the user is responsible to hand over correct datasets, but it would be nice to throw an appropriate error message for better comprehension.