Refactoring
- Rename dataloaders:
- closes https://github.com/kipoi/kipoiseq/issues/13
- `datasets.py` -> `dataloaders.py`
- `SeqDataset` -> `IntervalSeqDl`
- `SpliceDataset` -> `MMSpliceDl`
- remove `use_strand`
New features
- `MMSpliceDl` - MMSplice dataloader
- add `get_output_shape`
- Allow to use genomelake's `one_hot_encoding` when genomelake is installed