Hotfix 1.5.x - fix MNV coordinates

[julie-sullivan]

The MNV positions were incorrect. Here's an example:

ClinVar ID: 266834
chr13:32316508:GAC:ATCGATCGAT

Decomposed variants

Dragen	CellBase output
13:32316508:G:ATCGATCG	13:32316508::ATCGATC
13:32316510:C:T	13:32316517:C:T << wrong

The coordinates are calculated with the start, and the index:

genomic start + current index = start

Previously the code was using the index for the right aligned string, which was "-------GAC"

32316508 + 9 (index of -------GAC) = 32316517 << incorrect

I've updated the code to use the index of the original "GAC"

32316508 + 2 (index of GAC) = 32316510 << correct