Parse Strdrop for STR variants

[dnil]

Strdrop adds FORMAT tags (also Filter and Info, but let's ignore those).
FORMAT.DROP is the primary one, value '1' for drops, '0' otherwise. Suggest to highlight on STR view, perhaps as with pre-mutation. It will also be useful to parse, store and show the SDR, SDP and EDRvalues, e.g. on hover around GT on the variantS page, on the GT card together with or with hover on allele depths. Drops can perhaps be indicated in the "Expansion support collumn", which should be unused.

##INFO=<ID=STRDROP_P,Number=1,Type=Float,Description="Strdrop coverage sequencing depth level probability">
##INFO=<ID=STRDROP_EDR,Number=1,Type=Float,Description="Strdrop allele similarity Levenshtein edit distance ratio">
##INFO=<ID=STRDROP_SDR,Number=1,Type=Float,Description="Strdrop case average adjusted sequencing depth ratio">
##INFO=<ID=STRDROP,Number=0,Type=Flag,Description="Strdrop coverage drop detected">
##FILTER=<ID=LowDepth,Description="Strdrop coverage drop detected">
##FORMAT=<ID=SDP,Number=1,Type=Float,Description="Strdrop coverage sequencing depth level probability">
##FORMAT=<ID=EDR,Number=1,Type=Float,Description="Strdrop allele similarity Levenshtein edit distance ratio">
##FORMAT=<ID=SDR,Number=1,Type=Float,Description="Strdrop case average adjusted sequencing depth ratio">
##FORMAT=<ID=DROP,Number=0,Type=String,Description="Strdrop coverage drop detected, 1 for LowDepth">