Add cut generator abstraction

[blegat]

@odow @frapac @leclere I have added a cut generator abstraction as discussed. It is still rudimentary but the idea is there :-P We should add the RiskMeasure to AvgCutGenerator.

[codecov]

Codecov Report

Merging #8 into master will increase coverage by 4.03%.
The diff coverage is 91.3%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   84.66%   88.69%   +4.03%     
==========================================
  Files          12       13       +1     
  Lines         841      849       +8     
==========================================
+ Hits          712      753      +41     
+ Misses        129       96      -33

Impacted Files	Coverage Δ
src/nlds.jl	89.77% <100%> (+7.07%)	⬆️
src/node.jl	85% <100%> (+20%)	⬆️
src/sddp.jl	90.47% <100%> (+0.19%)	⬆️
src/sddptree.jl	90.9% <100%> (ø)	⬆️
src/comp.jl	100% <100%> (+10%)	⬆️
src/interface.jl	100% <100%> (ø)	⬆️
src/cutgen.jl	86.95% <86.95%> (ø)
... and 1 more

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[coveralls]

Coverage increased (+4.03%) to 88.693% when pulling 225df98 on cutgen into 8d64882 on master.

[coveralls]

Coverage increased (+4.03%) to 88.693% when pulling 225df98 on cutgen into 8d64882 on master.

[coveralls]

Coverage increased (+4.03%) to 88.693% when pulling 7bff6e2 on cutgen into 8d64882 on master.