Support GroupNormalization

[disktnk]

Trace chainer logic.

[codecov-io]

Codecov Report

Merging #219 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   89.99%   90.16%   +0.17%     
==========================================
  Files          24       24              
  Lines        1529     1556      +27     
==========================================
+ Hits         1376     1403      +27     
  Misses        153      153

Impacted Files	Coverage Δ
onnx_chainer/mapping.py	90% <ø> (ø)	⬆️
onnx_chainer/functions/array.py	90.55% <100%> (+0.03%)	⬆️
onnx_chainer/functions/normalization.py	90.47% <100%> (+4.03%)	⬆️
onnx_chainer/functions/__init__.py	100% <100%> (ø)	⬆️

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[shinh]

LGTM, thanks!