masked DNA strings

[kchu25]

There are some DNA strings in the datasets that either partially or entirely consist of masked strings, e.g., the 7th sequence in the DemoHumanOrWorm training set (checked via dset[6]), is a string of 'NNNNNNN....NNNN'. Maybe consider extracting the DNA strings from the unmasked genome?

[simecek]

I believe we use unmasked genome but I will look into that. It might still be that we hit the beginning / end of chromosomes that are often unknown. Maybe we should check the randomly chosen sequences and remove long all Ns.