Fix/issue with timestamp at point

[hrzn]

Solves #974
Caused by two underlying issues:

• The function TimeSeries.get_index_at_point() was not behaving as expected on integer-indexed series with index not starting at 0
• The historical forecasts was having an issue when building the resulting series (when last_points_only=True) with stride>1.

[codecov-commenter]

Codecov Report

Base: 93.73% // Head: 93.72% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (f478a13) compared to base (0597937).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1180      +/-   ##
==========================================
- Coverage   93.73%   93.72%   -0.01%     
==========================================
  Files          83       83              
  Lines        8398     8387      -11     
==========================================
- Hits         7872     7861      -11     
  Misses        526      526              

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.49% <ø> (ø)
darts/timeseries.py	92.24% <100.00%> (-0.05%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	87.45% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	98.55% <0.00%> (-0.02%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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