Fix: append_values and prepend_values correctly extends RangeIndex-typed TimeSeries

[mabilton]

Fixes #1434.

Summary

Previously, append_values and prepend_values did not correctly handle extending RangeIndex-typed TimeSeries. For example, the following code snippet:

from darts.utils.timeseries_generation import linear_timeseries
import numpy as np

series = linear_timeseries(start=1, length=5, freq=2)
print('Before `append_values`:')
print(list(series.time_index))
new_values = np.ones((1,))
print('After `append_values`:')
print(list(series.append_values(new_values).time_index))

would print:

Before `append_values`:
[1, 3, 5, 7, 9]
After `append_values`:
[0, 1, 2, 3, 4, 5]

This has been fixed in this PR so that this code now prints:

Before `append_values`:
[1, 3, 5, 7, 9]
After `append_values`:
[1, 3, 5, 7, 9, 11]

Similarly, the code snippet:

from darts.utils.timeseries_generation import linear_timeseries
import numpy as np

series = linear_timeseries(start=1, length=5, freq=2)
print('Before `prepend_values`:')
print(list(series.time_index))
new_values = np.ones((1,))
print('After `prepend_values`:')
print(list(series.prepend_values(new_values).time_index))

would previously print:

Before `append_values`:
[1, 3, 5, 7, 9]
After `append_values`:
[0, 1, 2, 3, 4, 5]

Now, this same code snippet prints:

Before `prepend_values`:
[1, 3, 5, 7, 9]
After `prepend_values`:
[-1, 1, 3, 5, 7, 9]

Other Information

Tests have also been updated to explicitly check for this behaviour.

[codecov-commenter]

Codecov Report

Base: 93.80% // Head: 93.80% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (1e17993) compared to base (9a40ca6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1435   +/-   ##
=======================================
  Coverage   93.80%   93.80%           
=======================================
  Files          94       94           
  Lines        9438     9423   -15     
=======================================
- Hits         8853     8839   -14     
+ Misses        585      584    -1     

Impacted Files	Coverage Δ
darts/timeseries.py	91.92% <ø> (+0.01%)	⬆️
...arts/models/forecasting/torch_forecasting_model.py	89.50% <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]

Great, thanks a lot 👍