Limit ambiguous dates by sample release date

## Description of proposed changes Limit year-only dates using upcoming precise date support in Augur (https://github.com/nextstrain/augur/issues/1494) Example:  <!-- What is the goal of this pull request? What does this pull request change? --> ## Related issue(s) - Depends on https://github.com/nextstrain/augur/issues/1494 ## Checklist <!-- Make sure checks are successful at the bottom of the PR. If applicable, add: - any changes to existing tests - any additional manual testing to confirm changes Please add a note if you need help with adding tests. --> - [ ] Add a column for `--target-date-field-min`? - [ ] Do the same for NCBI Datasets data? - [ ] Checks pass <!-- 🙌 Thank you for contributing to Nextstrain! ✨ -->