Series id support

[isaacmg]

The goal of this PR is to accomplish three things

• Support returning a series_id from the data-loader and the associated post-processing of data into models.
• Allow random or ordered sampling of different series for a specific datetime.
• Add support for multi-task data loader that allows the forecasting of many different locations or item codes all around the world

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.38%. Comparing base (71886b1) to head (9dc5ef0).
Report is 417 commits behind head on masterr.

Additional details and impacted files

@@           Coverage Diff            @@
##           masterr     #342   +/-   ##
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  Coverage    75.38%   75.38%           
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  Files           66       66           
  Lines         4537     4537           
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  Hits          3420     3420           
  Misses        1117     1117           

Flag	Coverage Δ
python	75.38% <0.00%> (ø)

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