add Matthews corrcoef

[SkafteNicki]

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #59
Second metric to be requested in the issue, Matthews correlation coefficient. Again, just a port of sklearn implementation.

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[codecov]

Codecov Report

Merging #98 (2fd7652) into master (84cd7ff) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ Coverage   97.23%   97.27%   +0.04%     
==========================================
  Files         136       70      -66     
  Lines        4270     2167    -2103     
==========================================
- Hits         4152     2108    -2044     
+ Misses        118       59      -59     

Flag	Coverage Δ
Linux	79.87% <100.00%> (+0.30%)	⬆️
Windows	79.87% <100.00%> (+0.30%)	⬆️
cpu	97.27% <100.00%> (+0.04%)	⬆️
gpu	?
macOS	97.27% <100.00%> (+0.04%)	⬆️
pytest	97.27% <100.00%> (+0.04%)	⬆️
python3.6	97.27% <100.00%> (+0.04%)	⬆️
python3.8	97.27% <100.00%> (+0.04%)	⬆️
python3.9	97.27% <100.00%> (+0.04%)	⬆️
torch1.3.1	95.69% <100.00%> (+0.06%)	⬆️
torch1.4.0	95.84% <100.00%> (+0.06%)	⬆️
torch1.8.0	97.27% <100.00%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
torchmetrics/__init__.py	100.00% <ø> (ø)
torchmetrics/classification/__init__.py	100.00% <100.00%> (ø)
torchmetrics/classification/matthews_corrcoef.py	100.00% <100.00%> (ø)
torchmetrics/functional/__init__.py	100.00% <100.00%> (ø)
torchmetrics/functional/classification/__init__.py	100.00% <100.00%> (ø)
...ics/functional/classification/matthews_corrcoef.py	100.00% <100.00%> (ø)
__w/2/s/torchmetrics/retrieval/retrieval_metric.py
.../torchmetrics/regression/mean_squared_log_error.py
__w/2/s/torchmetrics/metric.py
.../2/s/torchmetrics/regression/explained_variance.py
... and 66 more

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