move variationfeature fetches from slice to relevant adaptors
move variationfeature fetches from slice to relevant adaptors

ens-lgil/ensembl-rest,helensch/ensembl-rest,ens-lgil/ensembl-rest,willmclaren/ensembl-rest,helensch/ensembl-rest,ens-lgil/ensembl-rest,willmclaren/ensembl-rest,willmclaren/ensembl-rest,ens-lgil/ensembl-rest,helensch/ensembl-rest,ens-lgil/ensembl-rest,helensch/ensembl-rest,willmclaren/ensembl-rest,helensch/ensembl-rest,willmclaren/ensembl-rest