Add Korea to demographics dict

[jdebacker]

This PR updates demographics.py so that it can look to the UN Population Data repo for the country of Korea.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.12%. Comparing base (356087d) to head (50c38cf).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #998   +/-   ##
=======================================
  Coverage   70.12%   70.12%           
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  Files          20       20           
  Lines        5047     5047           
=======================================
  Hits         3539     3539           
  Misses       1508     1508           

Flag	Coverage Δ
unittests	70.12% <ø> (ø)

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Files with missing lines	Coverage Δ
ogcore/demographics.py	55.68% <ø> (ø)

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