New multi-file samplers, adjusts the selene.samplers API

In addition to adding a sampler that loads in .mat or .bed files for sampling in training/testing/validation modes (MultiFileSampler), we also have updated selene.sequences.Genome to include an input of blacklist_regions in its constructor. This allows users to specify whether certain regions of the genome should be ignored entirely (e.g. never get sampled when using an online sampler).