Adds drop_columns method to TimeSeries

[shaido987]

Fixes #880.

Summary

Adds a function to drop one or multiple columns to the TimeSeries. I tried follow the format and style of the other TimeSeries methods and also added a test to the tests/test_timeseries_multivariate file (since it does not make sense to drop columns in a single column time series).

[codecov-commenter]

Codecov Report

Merging #1040 (d26fb0f) into master (945ded5) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1040      +/-   ##
==========================================
- Coverage   91.32%   91.31%   -0.01%     
==========================================
  Files          77       77              
  Lines        7870     7862       -8     
==========================================
- Hits         7187     7179       -8     
  Misses        683      683              

Impacted Files	Coverage Δ
darts/timeseries.py	92.04% <100.00%> (-0.02%)	⬇️
darts/models/__init__.py	86.04% <0.00%> (-0.32%)	⬇️
...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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[hrzn]

LGTM, thanks!