Use pandas.DataFrame.at instead of .loc for single values

[victorlin]

Description of proposed changes

This is the recommended way by pandas:

Similar to loc, in that both provide label-based lookups. Use at if you only need to get or set a single value in a DataFrame or Series.

Related issue(s)

• Follow-up to utils: Remove read_metadata() in favor of io.read_metadata() #934
• Document how to upgrade read_metadata #970

Testing

No tests added.

[codecov]

Codecov Report

Base: 61.64% // Head: 61.64% // No change to project coverage 👍

Coverage data is based on head (740fc85) compared to base (f148234).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #979   +/-   ##
=======================================
  Coverage   61.64%   61.64%           
=======================================
  Files          52       52           
  Lines        6286     6286           
  Branches     1583     1583           
=======================================
  Hits         3875     3875           
  Misses       2139     2139           
  Partials      272      272           

Impacted Files	Coverage Δ
augur/export_v1.py	74.35% <100.00%> (ø)
augur/refine.py	63.13% <100.00%> (ø)

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[victorlin]

Merging without review since I think this is a trivial change, and patch coverage is 100% according to #979 (comment).