Support PMC identifier in Citation.

[jvwong]

fix(pubmedxmlparser): Add the PMCID, when available

Refs #41

[codecov]

Codecov Report

Merging #42 (ffbc9b2) into development (5f1af68) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development      #42      +/-   ##
===============================================
+ Coverage        92.80%   92.94%   +0.14%     
===============================================
  Files                5        5              
  Lines              250      255       +5     
===============================================
+ Hits               232      237       +5     
  Misses              18       18              

Impacted Files	Coverage Δ
ncbiutils/pubmedxmlparser.py	100.00% <100.00%> (ø)

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