Chinese pos tagset mapping

[apmoore1]

Fixes #19

1. Adds a mapping from the Penn Chinese Treebank POS tagset to USAS core POS tagset.
2. In the documentation it clarifies that we used the Universal Dependencies Treebank version of the UPOS tagset rather than the original version from the paper by Petrov et al. 2012.

[codecov-commenter]

Codecov Report

Merging #22 (e5e33bf) into main (8344730) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files           8        8           
  Lines         335      336    +1     
  Branches       66       66           
=======================================
+ Hits          327      328    +1     
  Misses          7        7           
  Partials        1        1           

Impacted Files	Coverage Δ
pymusas/pos_mapper.py	100.00% <100.00%> (ø)

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