Prevent padded intervals from exceeding chromosome ends

[avantikalal]

In LabeledSeqDataset, we add padding at the end of intervals to load extra bases for data augmentation by shift/jittering. Similarly in VariantDataset, we generate intervals surrounding the variant. In both these cases, the extended/generated intervals may end up extending beyond the chromosome which will result in an error when we try to read sequences.

This PR introduces the following changes:

1. adds a function check_chrom_ends which raises an error if any intervals exceed the chromosome ends, and prints the relevant intervals.
2. Added a test for this function
3. Added this function to the dataset classes.

Addresses #54