snp_filter: Minor fixes
snp_filter: Minor fixes

Clarify some of the logic for computing vcf_range. Remove problematic
asserts until we can fix compilation problems in debug mode.

NVlabs/firepony,NVlabs/firepony,NVlabs/firepony,broadinstitute/firepony,broadinstitute/firepony,NVlabs/firepony,broadinstitute/firepony,broadinstitute/firepony