Enable preferred sampling of specific regions during strain selection

[huddlej]

The subsampling logic for strain selection in original augur supported a "priority" sampling for specific regions. This type of prioritization is helpful for custom flu builds where one wants to investigate evolutionary patterns in specific regions. The current strain selection script for seasonal flu exposes the same --sampling flag used by old augur, but this flag is ignored.

To enable preferred sampling of regions by the current script, some of the code from the original subsampling script needs to be migrated.