Fix/int index warnings

[hrzn]

Remove Int64Index support and keep only RangeIndex support.

As a consequence, this means that TimeSeries cannot have missing dates anymore (which was possible before with Int64Index; RangeIndex do not have holes by construction).

I had to adapt

• TimeSeries.from_dataframe() to handle correctly time columns made of integers (and ensure they correctly map to a RangeIndex)
• The __getitem__() method, because sel() on a DataArray returns an array indexed with an Int64Index (see: Indexing a RangeIndexed' DataArray with a RangeIndex returns a deprecated Int64Index pydata/xarray#6256)
• The unit tests to make sure these changes are covered.

Some warnings remain for the time being because of statsmodels.

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Codecov Report

Merging #777 (33294f9) into master (508d60a) will increase coverage by 0.22%.
The diff coverage is 78.72%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
+ Coverage   90.72%   90.94%   +0.22%     
==========================================
  Files          68       68              
  Lines        6746     6803      +57     
==========================================
+ Hits         6120     6187      +67     
+ Misses        626      616      -10     

Impacted Files	Coverage Δ
darts/utils/data/encoders.py	96.69% <ø> (ø)
darts/utils/data/training_dataset.py	89.47% <ø> (ø)
darts/timeseries.py	88.02% <77.77%> (+1.99%)	⬆️
darts/utils/data/encoder_base.py	93.69% <100.00%> (ø)
darts/utils/timeseries_generation.py	95.49% <100.00%> (ø)
darts/metrics/metrics.py	100.00% <0.00%> (ø)

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[brunnedu]

Thanks for this important update! 🚀
Looks good to me, but it would be good if someone more familiar with xarray could take a look.