This is a temporary and hopefully short-lived solution. I've modified
the FunctionalClass stratification to stratify by effect impact as
defined by SnpEff annotations (high, moderate, and low impact) rather
than by the silent/missense/nonsense categories.
If we want to bring back the silent/missense/nonsense stratification,
we should probably take the approach of asking the SnpEff author
to add it as a feature to SnpEff rather than coding it ourselves,
since the whole point of moving to SnpEff was to outsource genomic
annotation.