Support precise date ranges

[victorlin]

Description of proposed changes

This has more flexibility than ambiguous dates in reduced precision which are limited to the range of a single calendar year or month.

Related issue(s)

• Closes Allow precise date ranges #1304

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests
• Check if you need to update docs
• Merge base PR Use standard date parsing logic for subsampling (and general improvements) #1775

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.64%. Comparing base (49197e0) to head (18422e7).

Files with missing lines	Patch %	Lines
augur/dates/__init__.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1770   +/-   ##
=======================================
  Coverage   73.63%   73.64%           
=======================================
  Files          81       81           
  Lines        8641     8650    +9     
  Branches     1765     1767    +2     
=======================================
+ Hits         6363     6370    +7     
- Misses       1976     1977    +1     
- Partials      302      303    +1     

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[joverlee521]

Changes LGTM. My only suggestion is to clearly document the supported formats for date intervals since that will dictate the outputs for augur curate format-dates in #1494

[victorlin]

Note: this format is unsupported during subsampling for the same reason as #844: subsampling doesn't use get_numerical_date_from_value, but instead has its own date parsing logic where only YYYY-MM-DD is supported. I'll look into swapping that with get_numerical_date_from_value.

[victorlin]

#1774 should be resolved before this is merged, so that the new format is supported by subsampling in augur filter.