Add metrics package

[r9y9]

fixes #12

Some highlights:

• Support for both torch and numpy arrays
• Support for mini-batch inputs

[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.96%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #45      +/-   ##
=========================================
+ Coverage   84.44%   85.4%   +0.96%     
=========================================
  Files          29      29              
  Lines        1414    1480      +66     
=========================================
+ Hits         1194    1264      +70     
+ Misses        220     216       -4

Impacted Files	Coverage Δ
nnmnkwii/metrics/__init__.py	100% <100%> (+100%)	⬆️

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