<DEL> symbolic variants imported in VCFs w/o SVLEN set to SVLEN positive

If we read in a VCF:

```
library.genomics.vcf_utils.vcf_get_ref_alt_svlen_and_modification
```

And it's symbolic but doesn't have SVLEN, we create it via:

```
elif end_info := variant.INFO.get('END'):
svlen = int(end_info) - variant.POS
```

This gives us a positive SVLEN for DEL

The scope of this issue is to get rid of dupes - get the import back to current standard which is SVLEN should be negative as matching 4.3

And finding and fixing any dupes

FUTURE WORK

* We may decide to make svlen always positive as per 4.4+ spec. However that can come later, best to remove dupes first