<Fix> evaluation dataset, printed samples

[hypothe]

Bunch of minor "theoretical" changes in the evaluation function:

1. test_data_gen was used as the data generator in the evaluation, instead of data_generator, thereby evaluating the net on the test set used for training (not an actual issue here given the sequences are randomized and not sampled from existing datasets, but in principle would lead to a data leak in realistic scenarios);
2. the correct sequences printed were a sampling (with reinsertion) of the first 10 evaluated, instead of 10 sampled from the whole set of correct ones;
3. the condition for printing the incorrectly classified sequences would declare the absence of misclassifications if verbose==False, independently of their actual presence;