Defines input-output maps with forecast variables ref

[Giovanni3A]

• Creates variable indexed vector and matrix
• Apply them on forecast step so predictions are indexed with variable refs
• Use the variable index when applying predicted values in plan step

[codecov]

Codecov Report

❌ Patch coverage is 78.52761% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.93%. Comparing base (bb71bba) to head (0a9d56e).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/variable_indexed_structs.jl	64.78%	25 Missing ⚠️
src/ApplicationDrivenLearning.jl	42.85%	4 Missing ⚠️
src/predictive_model.jl	92.30%	4 Missing ⚠️
src/optimizers/gradient.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #50      +/-   ##
==========================================
- Coverage   65.59%   64.93%   -0.66%     
==========================================
  Files          12       13       +1     
  Lines         558      693     +135     
==========================================
+ Hits          366      450      +84     
- Misses        192      243      +51     

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[Giovanni3A]

Closing this pull request because we though about a simpler way of doing this that results in lower training time.