anchor ARIMAS simulation to end of series

[dennisbader]

Fixes #1892

Summary

• fixes anchor for ARIMA simulation (probabilistic forecasting) from start to end to properly use future covariates

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (4ded99a) 93.93% compared to head (c23a76f) 93.92%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1893      +/-   ##
==========================================
- Coverage   93.93%   93.92%   -0.01%     
==========================================
  Files         125      125              
  Lines       11743    11729      -14     
==========================================
- Hits        11031    11017      -14     
  Misses        712      712              

Impacted Files	Coverage Δ
darts/datasets/__init__.py	100.00% <ø> (ø)
darts/models/forecasting/arima.py	91.48% <ø> (ø)
darts/timeseries.py	92.46% <100.00%> (-0.05%)	⬇️

... and 4 files with indirect coverage changes

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