[ADAM-416] Removing 'ADAM' prefix

This initial commit includes just the change for the ADAMFeaturesContext
class/object.

I'll add more commits (and accept PRs to this PR!) which remove the prefix from other classes.

Fixes #416.
I would actually _not_ rename this file, to make it obvious that this is distinct from the VCF output formats provided by Hadoop-BAM.