Scaffolding for BQSR implementation
add Util.hashCombine
add stuff to Observation
Make ObservationTable ctor private
ObservationTable fixes/changes
Empirical error probability tweaks
Don't need to broadcast SnpTable; it's only referenced once
add Recalibrator skeleton
Rename PhredQualityScore to QualityScore
ObservationTable fixes
ObservationTable doesn't need to be sorted
adjust variable naming
split out readGroup and quality covariates
Clean up covariates a bit
Compute and apply RecalibrationTable
Compute covariate values read-at-a-time to support contextual covariates