Variant importer should create a separate record for every alternate allele

Currently, alternate allele data, including sequences and coverage are being truncated for variant rows with multiple alternate alleles.  As an interim measure, this can be mitigated by treating every alternate allele as a variant in itself, and adding a separate row to the variants table for it.

This would require an additional foreign key on the variant_result table, allowing samples that are heterozygous alternate at a given locus to point to a variant record for each allele.

The allelic depth fields should be renamed, as they would no longer be dedicated to ref and alt, but would rather refer to their respective homologous sequences at a given locus.
Move this comment into the property definition:

```python
@property
def coverage_ref(self):
    """
    The ref allelic depth if it is available.
    """
    ...
```