Cache populations for stop in weighted avg svc freq
Use TrieMap rather than normal mutable map
Fix WSF unit label to minutes and fix accuracy
Merge branch 'moradology-feature/cache-popInStop' into feature/fix-wsf-unit-and-accuracy
Default sample period to second available date in range.

This is to avoid potential timezone issues if user is in different timezone than the GTFS.
Closes #387.
Merge pull request #395 from WorldBank-Transport/feature/fix-wsf-unit-and-accuracy

Fix WSF unit and accuracy
Update Google transit feed validator version.

Closes #396.
Merge pull request #397 from flibbertigibbet/feature/default-sample-period-second-day

Default sample period to second available date in range.
Merge pull request #398 from flibbertigibbet/feature/update-feed-validator

Update Google transit feed validator version.