Feature: Add support for logistic growth to Prophet

[DavidKleindienst]

Fixes #628 and #178

Summary

Adds support for logistic growth to Prophet and a corresponding unit-test.

This PR adds the function Prophet.set_capacity to allow setting cap and floor parameters needed.
Prophet.set_capacity takes two arguments (cap and floor), which may be either numbers or Callables.
floor defaults to 0, to replicate FBProphet's default behaviour.
Numbers should be used as arguments when modeling a constant carrying capacity, Callables can be supplied when variable carrying capacities are needed.
A Callable supplied as argument to Prophet.set_capacity must accept a DatetimeIndex or RangeIndex as input, and return a Sequence of numbers with the same length. Each number in this Sequence indicates the carrying capacity at the corresponding index.

During Prophet.fit and Prophet.predict cap and floor columns with the appropriate values are added to fit_df and predict_df, respectively.
If Prophet was instantiated with growth = 'logistic, Prophet.fit now raises a ValueError if carrying capacities have not been set.
If Prophet.set_capacity is called when growth is not logistic, a warning is logged and the cap and floor values set are ignored.

[codecov-commenter]

Codecov Report

Base: 94.09% // Head: 94.02% // Decreases project coverage by -0.07% ⚠️

Coverage data is based on head (a6fc312) compared to base (e1c8d34).
Patch coverage: 82.60% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1419      +/-   ##
==========================================
- Coverage   94.09%   94.02%   -0.07%     
==========================================
  Files         125      125              
  Lines       11071    11079       +8     
==========================================
  Hits        10417    10417              
- Misses        654      662       +8     

Impacted Files	Coverage Δ
darts/models/forecasting/prophet_model.py	92.59% <82.60%> (-1.70%)	⬇️
darts/utils/data/tabularization.py	99.27% <0.00%> (-0.73%)	⬇️
darts/timeseries.py	92.14% <0.00%> (-0.23%)	⬇️
darts/ad/anomaly_model/filtering_am.py	91.93% <0.00%> (-0.13%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.52% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.27% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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[hrzn]

Many thanks @DavidKleindienst and I'm really sorry it took us so long to review!

I think it looks really good. I would just prefer if we could specify cap and floor in the constructor instead of relying on an extra set_capacity() method. Let me know your thoughts.

[hrzn]

You should add a blank line for the bullet-point list to render correctly in HTML (you can try building & rendering the doc with ./gradlew buildDocs to check the HTML files directly).

[hrzn]

LGTM, thanks @DavidKleindienst !
There's only a linting issue remaining

[DavidKleindienst]

Fixed the linting problem and removed a the previous docstring that accidentally remained there.
The remaining failing tests seem to be unrelated.
Thanks @hrzn for reviewing!