fix quantile detector when low/high threshold are the same

[julien12234]

Fixes #1551. Fix the issue raised by philippspengler:

Describe the bug
The QuantileDetector class raises an error during fitting when the time series has low variation.

To Reproduce

series = TimeSeries.from_series(pd.Series([96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 96.0, 93.0]))
quantile_detector = QuantileDetector(low_quantile=0.05, high_quantile=0.95)
quantile_detector.fit(series)
Expected behavior
Expect the quantile detector to fit without an error.

System (please complete the following information):

Python version: 3.8.5
darts version 0.23.1

Solution:

The problemed occurred when the low and high quantiles corresponded to the same number, and when calling the threshold detector, an error was raised as it expected two different values. The condition was removed.

[dennisbader]

👍 Nice, thanks! Could you also add a unit test to check that?

Btw you can reference issues like this: #1551
And you can write "fixes #1551" in your PR description to automatically close the issue as soon as the PR is merged :)

[codecov-commenter]

Codecov Report

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

Coverage data is based on head (d4dd59e) compared to base (e1c8d34).
Patch coverage: 85.18% of modified lines in pull request are covered.

❗ Current head d4dd59e differs from pull request most recent head 57d8ab2. Consider uploading reports for the commit 57d8ab2 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1553      +/-   ##
==========================================
- Coverage   94.09%   94.02%   -0.07%     
==========================================
  Files         125      125              
  Lines       11071    11081      +10     
==========================================
+ Hits        10417    10419       +2     
- Misses        654      662       +8     

Impacted Files	Coverage Δ
darts/ad/detectors/quantile_detector.py	100.00% <ø> (ø)
darts/ad/detectors/threshold_detector.py	100.00% <ø> (ø)
darts/models/forecasting/prophet_model.py	92.59% <82.60%> (-1.70%)	⬇️
darts/ad/scorers/kmeans_scorer.py	100.00% <100.00%> (ø)
darts/datasets/__init__.py	100.00% <100.00%> (ø)
darts/utils/timeseries_generation.py	96.15% <100.00%> (ø)
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%)	⬇️
... and 3 more

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[dennisbader]

Looks good! Thanks 🚀