dates: Use same date parsing functions for both CLI arguments and metadata

[victorlin]

Description of proposed changes

Replace get_numerical_date_from_value() with a new function wrapping any_to_numeric() that can return (min, max) for non-exact dates.

This ensures that the same date formats are supported across Augur to avoid issues like #747, though I wouldn't expect relative dates to be used in metadata (but the support will be there).

Tasks:

• merge dates: Move date logic from utils to dates #923
• merge dates: Rewrite using regular expressions #928
• do work here

Related issue(s)

• Will fix Incomplete dates are not interpreted as ambiguous by augur filter #747
• Will fix Augur filter does not support the integer.float input date format for --min/max-date filtering #748

Testing

TODO

[victorlin]

Closing in favor of #1144