Split iteration in forwardpass and backwardpass

[blegat]

Implements @frapac's idea in blegat/PseudoSDDP#1

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   87.88%   87.93%   +0.04%     
==========================================
  Files          14       14              
  Lines         867      870       +3     
==========================================
+ Hits          762      765       +3     
  Misses        105      105

Impacted Files	Coverage Δ
src/sddp.jl	83.33% <100%> (+0.57%)	⬆️

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[coveralls]

Coverage increased (+0.04%) to 87.931% when pulling 6b64161 on split into f5775c6 on master.