Add mouseover tooltip to explain custom fields in the genotype table header for VariantFeatureDetails

[cmdcolin]

Some values in the VCF FORMAT/genotype table are custom strings or values

example: OBS in a Jannovar VCF

##FORMAT=<ID=OBS,Number=A,Type=String,Description="Posterior odds for alt allele of each fragment as Kass Raftery scores: N=none, B=barely, P=positive, S=strong, V=very strong (lower case if probability for correct mapping of fragment is <95%)">

we can create a mouseover tooltip to try to help explain these