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[bot]]

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%)	:arrow_up:
src/node.jl	85% <100%> (+20%)	:arrow_up:
src/sddp.jl	90.47% <100%> (+0.19%)	:arrow_up:
src/sddptree.jl	90.9% <100%> (ø)	:arrow_up:
src/comp.jl	100% <100%> (+10%)	:arrow_up:
src/interface.jl	100% <100%> (ø)	:arrow_up:
src/cutgen.jl	86.95% <86.95%> (ø)
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8d64882...7bff6e2. Read the comment docs.

[coveralls]

Coverage increased (+4.03%) to 88.693% when pulling 225df98cf8f3516d6ae733d10932c4c4dffb7d38 on cutgen into 8d6488281c53e3c82a1184f90568cc11e8da42ed on master.

[coveralls]

Coverage increased (+4.03%) to 88.693% when pulling 225df98cf8f3516d6ae733d10932c4c4dffb7d38 on cutgen into 8d6488281c53e3c82a1184f90568cc11e8da42ed on master.

[coveralls]

Coverage increased (+4.03%) to 88.693% when pulling 7bff6e2b9594a1f76d4bea1b6bbbda21a671012c on cutgen into 8d6488281c53e3c82a1184f90568cc11e8da42ed on master.