prepare for new mutation data format from server & clean up closestNode()
prepare for new mutation data format from server
& clean up closestNode()

ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,acthp/ucsc-xena-client,acthp/ucsc-xena-client,acthp/ucsc-xena-client