Add beginnings of a deterministic equivalent solver

[odow]

Still to do

• tests
• documentation
• check for cyclic policy graph
• limit on size? This thing might be huge. Maybe a time limit to build the model.

[codecov]

Codecov Report

Merging #229 into master will increase coverage by 0.46%.
The diff coverage is 98.96%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   93.27%   93.73%   +0.46%     
==========================================
  Files          12       13       +1     
  Lines        1101     1198      +97     
==========================================
+ Hits         1027     1123      +96     
- Misses         74       75       +1

Impacted Files	Coverage Δ
src/SDDP.jl	100% <ø> (ø)	⬆️
src/deterministic_equivalent.jl	98.96% <98.96%> (ø)

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