use modifier to speed up LD calls
Created by: willmclaren
This modifier tells the variation API to not fetch VariationFeature objects, but rather rely on the position->ID mapping extracted from the VCF during genotype extraction. This should make these REST calls many times faster (5-8x faster in my tests).
It depends on a set of changes in ensembl-variation master for the speedup, but safely falls back to the old behaviour when using release/83.