adding DS tag that is the score that should be used to select the
non-duplicate among duplicates
adding a function to give the cannonical read name, defined as the
concatenation of the read group identifier and the read name.
adding a class that stores information related to and methods that hep
with the scoring strategy for when to choose non-duplicates among a set
of comparable records (same 5' unclipped position etc.).
allow the duplicate scoring strategy to be set in the SAM builder