feat: add mid-point rooting to refine

[rneher]

Description of proposed changes

Add option to mid-point root to refine, both with and without timetree.

Checklist

• Add a message in CHANGES.md summarizing the changes in this PR that are end user focused. Keep headers and formatting consistent with the rest of the file.

[codecov]

Codecov Report

Patch coverage: 14.28% and project coverage change: -0.05% ⚠️

Comparison is base (8bc8b0d) 69.85% compared to head (d963542) 69.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1257      +/-   ##
==========================================
- Coverage   69.85%   69.80%   -0.05%     
==========================================
  Files          67       67              
  Lines        7155     7160       +5     
  Branches     1740     1742       +2     
==========================================
  Hits         4998     4998              
- Misses       1851     1855       +4     
- Partials      306      307       +1     

Files Changed	Coverage Δ
augur/refine.py	64.28% <14.28%> (-1.57%)	⬇️

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[huddlej]

Worked for me! I included an example below based on the Zika tutorial.

Root on midpoint:

Root on best: