utils: Fix numeric date handling

[victorlin]

Description of proposed changes

Currently, date format handling is inaccurate (#747) as numeric dates are thrown out:

augur/augur/utils.py

Lines 115 to 119 in a85194c

	if type(value)!=str:
	if raise_error:
	raise ValueError(value)
	else:
	numerical_date = None

This change ensures numeric dates are processed, and that non-negative integers are evaluated as year-only ambiguous dates.

Also including a few refactors:

• Remove raise_error parameter. The intent is unclear and tests still pass without it.
• Use return instead of an intermediate variable.

Related issue(s)

Fixes #747.

Testing

• Add broken tests and verify new changes pass.
• Fix inaccurate existing tests.

[codecov]

Codecov Report

Merging #841 (bfffeca) into master (a85194c) will increase coverage by 0.07%.
The diff coverage is 78.57%.

❗ Current head bfffeca differs from pull request most recent head bfb21da. Consider uploading reports for the commit bfb21da to get more accurate results

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
+ Coverage   33.83%   33.91%   +0.07%     
==========================================
  Files          41       41              
  Lines        5905     5906       +1     
  Branches     1507     1507              
==========================================
+ Hits         1998     2003       +5     
+ Misses       3824     3821       -3     
+ Partials       83       82       -1     

Impacted Files	Coverage Δ
augur/utils.py	42.89% <78.57%> (+1.30%)	⬆️

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[huddlej]

Thanks, @victorlin! This looks great.