fixed wrong future covariates slicing with RangeIndex

[dennisbader]

Fixes #843

Summary

• fixed wrong RangeIndex-based future covariates slicing for DualCovariatesForecastingModels
• added range index tests for LocalForecastingModels

[codecov-commenter]

Codecov Report

Merging #858 (2ad425d) into master (0233756) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
+ Coverage   91.37%   91.40%   +0.02%     
==========================================
  Files          70       70              
  Lines        7105     7106       +1     
==========================================
+ Hits         6492     6495       +3     
+ Misses        613      611       -2     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.78% <100.00%> (+0.01%)	⬆️
darts/models/forecasting/prophet_model.py	94.24% <0.00%> (+0.71%)	⬆️
darts/models/forecasting/auto_arima.py	100.00% <0.00%> (+3.57%)	⬆️

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[hrzn]

Good catch, thanks!