Add support for displaying SNPs in "no_ref" CRAM files

[cmdcolin]

CRAM can be written as a "no_ref" file which encodes the deltas against the reference genome in the MD tag instead of the "CRAM encoded read features"

the noref files are a little larger but they are a default on galaxy

140K    volvox-sorted.cram
228K    volvox-sorted.noref.cram
388K    volvox-sorted.bam

cc @alpapan