Allow users to specify VQSLOD sensitivity and apply threshold in ExtractCohort

[mmorgantaylor]

specops issue #269 https://github.com/broadinstitute/dsp-spec-ops/issues/269

• user can provide either [snps-truth-sensitivity-filter-level, indels-truth-sensitivity-filter-level] or [snps-lod-score-cutoff, indels-lod-score-cutoff], or neither, in which case default values of snps-truth-sensitivity-filter=99.7 and indels-truth-sensitivity-fitler=99.0 are used.
• regardless of lod score cutoffs or truth sensitivity cutoffs, the filtering string is either "low_VQSLOD_SNP" or "low_VQSLOD_INDEL"
• if lod score cutoffs are provided, those are used for filtering. the header filter message looks like "Site failed INDEL model VQSLOD cutoff of 0.0"
• if truth sensitivity cutoffs are provided, the corresponding lod scores are looked up from the tranche table in BigQuery. the header filter message in this case looks like "Site failed INDEL model sensitivity cutoff (90.0), corresponding with VQSLOD cutoff of 0.0"

[kcibul]

looks great