add already existing forecasts param

[JanFidor]

Fixes #1283

Summary

For now it's just a rough implementation, once it's green lit by @dennisbader i'll add some tests

[dennisbader]

Thanks @JanFidor for this!

Just one comment about the missing historical_forecasts defnition in the docs.
After that it's ready to be merged!

[dennisbader]

Could you add the parameter definition to the docstring?

[JanFidor]

On it

[JanFidor]

Hi @dennisbader, I have 3 quick questions about the PR:

1. Should I add 1 or 2 tests for the param in test_backesting.py?
2. Right now the forecasts param is second which imho makes sense when it comes to readability, but would be a breaking change when not using named params. Do you think it's worth it to stay with the more readable implementation?
3. I was thinking about potential edge cases while browsing the file and noticed this line should I do something similar, but maybe instead of throwing an exception, log a warning and fallback on calling historical_forecasts() ?

[JanFidor]

@dennisbader I added the docstring, so everything should be ready for merging, if you think any of my proposed changes from the previous comment would be useful to the PR just let me know and I'll get on them!

[dennisbader]

Hey @JanFidor , thanks again for another nice PR :)

Had only some minor comments.

A small unittest would be nice (maybe to existing backtest tests) to check wether we get identical results with and without provided historical_forecasts.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.08 ⚠️

Comparison is base (f7c292a) 94.13% compared to head (f2156fa) 94.05%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1597      +/-   ##
==========================================
- Coverage   94.13%   94.05%   -0.08%     
==========================================
  Files         125      125              
  Lines       11340    11328      -12     
==========================================
- Hits        10675    10655      -20     
- Misses        665      673       +8     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.87% <100.00%> (+0.01%)	⬆️

... and 9 files with indirect coverage changes

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[dennisbader]

Hi @JanFidor and thanks for the updates and patience!

Only had minor suggestions mainly for rephrasing the docs.
After that we can merge this one 🚀

[JanFidor]

Hi @dennisbader , thanks for the reviews and help with this PR! Special thanks for the suggested changes to the docs, writing them is still not my strong suit and something I'll have to keep working on ; )

[dennisbader]

LGTM, thanks @JanFidor 🚀