Feat/improve integer indexes

[hrzn]

Solves #974 and fixes a series of small issues we had with integer-indexed series:

• The function TimeSeries.get_index_at_point() was not behaving as expected on integer-indexed series with index not starting at 0
• The historical forecasts was having an issue when building the resulting series (when last_points_only=True) with stride>1.
• The time index of forecasts and a few other things were basically incorrect when the underlying RangeIndex was having a step > 1.
• The TimeSeries.gaps() method had an issue with integer-indexed series.
• Some unit tests were missing to spot these issues
• Added unit tests for RegressionModels with integer series. Previously part of the logic was relying on [] indexing, which would have failed for series not starting at 0 or with step > 1. Fixes Feat/improve integer indexes #1191

This PR supersedes #1180, which I'll now close.

[codecov-commenter]

Codecov Report

Base: 93.71% // Head: 93.77% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (3d105d6) compared to base (a162e21).
Patch coverage: 97.22% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1191      +/-   ##
==========================================
+ Coverage   93.71%   93.77%   +0.05%     
==========================================
  Files          83       83              
  Lines        8416     8414       -2     
==========================================
+ Hits         7887     7890       +3     
+ Misses        529      524       -5     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.49% <ø> (ø)
darts/timeseries.py	92.34% <95.23%> (+0.31%)	⬆️
darts/models/forecasting/regression_model.py	98.33% <100.00%> (+0.65%)	⬆️
darts/utils/data/encoder_base.py	93.69% <100.00%> (ø)
darts/utils/timeseries_generation.py	95.86% <100.00%> (+0.03%)	⬆️
...arts/models/forecasting/torch_forecasting_model.py	87.45% <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)
... and 1 more

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[dennisbader]

Very nice and thanks a lot! These are very important changes.
I tested most of the changes and added some minor comments.

What do you think in general of adding some examples (not in this PR) for the most of "get" methods for documentation? Could be helpful for the user as some of these methods have quite different behavior depending on input and underlying series.

[dennisbader]

As getitem gets more complex, I believe now would be a good point either to add examples for all cases (probably better) or a userguide. Would probably make more sense in another PR.
What do you think?

[hrzn]

Absolutely agree! #1227