Add belief states

[odow]

Implements the algorithm outlined in http://www.optimization-online.org/DB_HTML/2019/03/7141.html

[codecov]

Codecov Report

Merging #192 into master will decrease coverage by 0.24%.
The diff coverage is 92.14%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
- Coverage   92.37%   92.12%   -0.25%     
==========================================
  Files          11       11              
  Lines         905     1067     +162     
==========================================
+ Hits          836      983     +147     
- Misses         69       84      +15

Impacted Files	Coverage Δ
src/plugins/sampling_schemes.jl	87.8% <ø> (ø)	⬆️
src/plugins/bellman_functions.jl	93.78% <100%> (ø)	⬆️
src/plugins/risk_measures.jl	94.05% <81.81%> (-2.75%)	⬇️
src/algorithm.jl	95.52% <89.41%> (-1.84%)	⬇️
src/user_interface.jl	97.15% <95.69%> (-0.99%)	⬇️

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