properly handle passed axis in TimeSeries.plot

[dennisbader]

Fixes #1911.

Summary

• fixes issue where stochastic TimeSeries were not plotted properly when passing a custom ax to TimeSeries.plot()
• improve plotting to rely on axis rather than pyplot and fig
• return axis

[codecov-commenter]

Codecov Report

Patch coverage: 92.60% and project coverage change: -0.12% ⚠️

Comparison is base (3376f27) 93.82% compared to head (adb51ee) 93.71%.
Report is 3 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1916      +/-   ##
==========================================
- Coverage   93.82%   93.71%   -0.12%     
==========================================
  Files         126      131       +5     
  Lines       12214    12648     +434     
==========================================
+ Hits        11460    11853     +393     
- Misses        754      795      +41     

Files Changed	Coverage Δ
darts/utils/__init__.py	100.00% <ø> (ø)
darts/utils/timeseries_generation.py	96.15% <ø> (ø)
darts/utils/utils.py	80.58% <ø> (-11.09%)	⬇️
darts/explainability/__init__.py	75.00% <71.42%> (-25.00%)	⬇️
darts/timeseries.py	92.66% <81.81%> (+0.14%)	⬆️
darts/models/forecasting/forecasting_model.py	95.21% <84.00%> (+0.07%)	⬆️
...orical_forecasts/optimized_historical_forecasts.py	88.46% <88.46%> (ø)
darts/explainability/explainability_result.py	92.13% <90.00%> (-2.87%)	⬇️
darts/explainability/shap_explainer.py	87.69% <91.66%> (-1.66%)	⬇️
darts/explainability/tft_explainer.py	93.15% <93.15%> (ø)
... and 8 more

... and 4 files with indirect coverage changes

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[madtoinou]

LGTM 🚀