Changes to stats given filters & multiple samples

Two main changes here:

  1. When there are multiple samples, I scale up the truth counts. We could be smarter about this, but this is at least defensible and guarantees that we show False Negative >= 0.
  1. We apply only a sensible subset of filters to the validation data. In this case, that's range restrictions & ref/alt filters.

The `LEFT JOIN` proved to be unnecessary, though we'll have to revive it if we want to label TP/FP in the table.

Fixes #329