Propagate static covs and hierarchy in missing value filler

[hrzn]

Solves #1075

[codecov-commenter]

Codecov Report

Merging #1076 (ce351f6) into master (8b6d1ab) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1076      +/-   ##
==========================================
- Coverage   93.16%   93.15%   -0.02%     
==========================================
  Files          78       78              
  Lines        7948     7935      -13     
==========================================
- Hits         7405     7392      -13     
  Misses        543      543              

Impacted Files	Coverage Δ
darts/utils/missing_values.py	96.96% <100.00%> (ø)
darts/timeseries.py	92.14% <0.00%> (-0.07%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.51% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.14% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.25% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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[dennisbader]

Looks great, thanks for quickly fixing these issues with static covs :)