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

Merging #342 (9dc5ef0) into master (71886b1) will not change coverage.
The diff coverage is n/a.

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

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

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