Adjacent variants crash svaha2

[edawson]

I have repeatedly run into this issue so it's high time I write it down. Variants in a vcf with adjacent positions will cause a segfault, probably because of some sloppy pointer invalidation I coded in mid-thesis.

The right way to fix this is to make a map from genomic position -> a list of alleles/nodes, rather than a single entity. This increases the code complexity but would be a useful improvement.