add mat2json tools

[jin-s13]

#123

[innerlee]

could you add some examples in doc?

[innerlee]

ideally also in test

[codecov]

Codecov Report

Merging #126 into master will increase coverage by 0.30%.
The diff coverage is 74.07%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   80.81%   81.11%   +0.30%     
==========================================
  Files          83       83              
  Lines        4685     4760      +75     
  Branches      822      840      +18     
==========================================
+ Hits         3786     3861      +75     
+ Misses        761      757       -4     
- Partials      138      142       +4     

Flag	Coverage Δ
#unittests	81.11% <74.07%> (+0.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ts/datasets/top_down/topdown_onehand10k_dataset.py	57.27% <ø> (ø)
mmpose/core/evaluation/eval_hooks.py	66.66% <67.24%> (+37.63%)	⬆️
mmpose/apis/inference.py	77.16% <91.30%> (+2.16%)	⬆️
mmpose/datasets/pipelines/top_down_transform.py	78.18% <0.00%> (-2.43%)	⬇️
mmpose/models/detectors/top_down.py	74.45% <0.00%> (+0.72%)	⬆️
mmpose/apis/test.py	27.90% <0.00%> (+13.95%)	⬆️

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