fix getitem with rangeindex start != 0 and freq != 1

[dennisbader]

Summary

• fixes TimeSeries __getitem__ with series that have a RangeIndex with start != 0 and freq != 1.
• adds unit test for getitem of integer indexed series

[codecov-commenter]

Codecov Report

Patch coverage: 83.33% and project coverage change: -0.14 ⚠️

Comparison is base (b668a4e) 94.29% compared to head (ed84770) 94.16%.

❗ Current head ed84770 differs from pull request most recent head 49c2d4f. Consider uploading reports for the commit 49c2d4f to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1868      +/-   ##
==========================================
- Coverage   94.29%   94.16%   -0.14%     
==========================================
  Files         125      125              
  Lines       11636    11627       -9     
==========================================
- Hits        10972    10948      -24     
- Misses        664      679      +15     

Impacted Files	Coverage Δ
darts/timeseries.py	92.46% <ø> (-0.22%)	⬇️
...sing/transformers/static_covariates_transformer.py	97.36% <83.33%> (-1.96%)	⬇️

... and 9 files with indirect coverage changes

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[madtoinou]

LGTM, congratulation for spotting this bug 🚀