change order of arguments for metric in gridsearch

[ClaraGrthns]

Fixes #979.

Summary

Metric in gridsearch method of the forecasting class now receives correct order of arguments: First the actual series, second the predicted/fitted series.

[hrzn]

Nice, thanks!
Could you maybe change the docstring of the gridsearch and backtest methods to make this clear?

Something like

metric
            A function that takes two TimeSeries instances as inputs (actual and prediction, in this order), and returns a float error value.

[codecov-commenter]

Codecov Report

Merging #989 (ea86f6e) into master (a1328fa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #989   +/-   ##
=======================================
  Coverage   93.04%   93.04%           
=======================================
  Files          77       77           
  Lines        7861     7861           
=======================================
  Hits         7314     7314           
  Misses        547      547           

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.77% <100.00%> (ø)

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[hrzn]

@ClaraGrthns are you still willing to help us close this PR? We can do it as soon as the docstrings are updated.

[ClaraGrthns]

Yes, I am sorry! I will do it next Tuesday!

[hrzn]

Thank you @ClaraGrthns
Do you agree to be mentioned / acknowledged in the changelog?

[hrzn]

@ClaraGrthns the new docstring line is too long, which causes the linting checks to fail. Could you break the line? Thanks.

[ClaraGrthns]

Just added the line break.

Thank you @ClaraGrthns Do you agree to be mentioned / acknowledged in the changelog?

yes, sure!