Update coloring for graphs

[colin99d]

Summary

Currently the charting in Darts messes up if you are using custom settings in matplotlib. I am creating a default colors argument so that users can choose whether they want Dart's default colors. If this is a desired feature I can add it to the rest of the charting functions in this file, and any other files you point out.

[hrzn]

Thanks for this one. I think we could add it. Could you also maybe do the same for the plot() function in TimeSeries ? Also we could maybe think of it as "default formatting" instead of limiting it to colors only, WDYT?

The tests are failing because of black - could you apply black and update?

[colin99d]

Thanks for the feedback. All requested changes have been implemented.

[hrzn]

Thanks!

[codecov-commenter]

Codecov Report

Merging #1056 (87d15eb) into master (8b6d1ab) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1056      +/-   ##
==========================================
- Coverage   93.16%   93.16%   -0.01%     
==========================================
  Files          78       78              
  Lines        7948     7941       -7     
==========================================
- Hits         7405     7398       -7     
  Misses        543      543              

Impacted Files	Coverage Δ
darts/timeseries.py	92.14% <100.00%> (-0.07%)	⬇️
darts/utils/statistics.py	88.65% <100.00%> (+0.36%)	⬆️
...arts/models/forecasting/torch_forecasting_model.py	89.51% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.14% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.25% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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