This milestone will be completed when GVL supports returning spliced sequences. The primary use case for this is working with personalized exonic sequences.

Partially addresses #24.

Implementation notes
When writing the dataset, this could be enabled by a splice=True flag and providing a BED file with named regions. Having unnamed regions should raise an error or warning. Regions with the same name will be spliced together in the order that they appear during reconstruction. e.g. gvl.Dataset[0] returns a spliced sequence corresponding to the first name appearing in the BED file. This will probably look like calling the reconstruct_haplotype function multiple times, once for each subsequence. This may also require disambiguating between regions and spliced regions in the documentation and/or gvl.Dataset attribute names.