Fix issue #659

[cprudhom]

Fixes # 659.

Changes proposed in this PR:

• Adding two methods in Solver, namely moveForward(Decision) and moveBackward to make possible applying a step-by-step search space exploration.

Example:

Model model = new Model();                               
IntVar x1 = model.intVar("x1", 0, 2);                    
IntVar x2 = model.intVar("x2", 0, 2);                    
x1.eq(x2).post();                                        
Solver solver = model.getSolver();                       
IntStrategy strategy = inputOrderLBSearch(x1, x2);       
Decision<IntVar> dec = null;         
int sol = 0;                         
boolean search = true;               
while(search) {                      
    if (solver.moveForward(dec)) {   
        dec = strategy.getDecision();
        if (dec == null) {           
            sol++;                   
        }else {                      
            continue;                
        }                            
    }                                
    search = solver.moveBackward();  
}                                    
System.out.printf("# solutions : %d\n", sol);

[codecov-io]

Codecov Report

Merging #660 into master will decrease coverage by 22.92%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             master     #660       +/-   ##
=============================================
- Coverage      62.7%   39.77%   -22.93%     
+ Complexity    10309     6779     -3530     
=============================================
  Files           658      658               
  Lines         42712    42747       +35     
  Branches       7807     7811        +4     
=============================================
- Hits          26782    17003     -9779     
- Misses        13603    24045    +10442     
+ Partials       2327     1699      -628

Impacted Files	Coverage Δ	Complexity Δ
...r/src/main/java/org/chocosolver/solver/Solver.java	63.15% <0%> (-19.81%)	85 <0> (-23)
...r/parser/flatzinc/ast/declaration/Declaration.java	0% <0%> (-100%)	0% <0%> (-1%)
.../chocosolver/solver/search/limits/NodeCounter.java	0% <0%> (-100%)	0% <0%> (-3%)
...solver/constraints/set/PropIntCstNotMemberSet.java	0% <0%> (-100%)	0% <0%> (-5%)
...ava/org/chocosolver/solver/learn/Implications.java	0% <0%> (-100%)	0% <0%> (-1%)
.../chocosolver/solver/objective/ParetoOptimizer.java	0% <0%> (-100%)	0% <0%> (-15%)
...r/solver/constraints/extension/TupleValidator.java	0% <0%> (-100%)	0% <0%> (-1%)
...olver/constraints/extension/binary/PropBinAC3.java	0% <0%> (-100%)	0% <0%> (-15%)
...ava/org/chocosolver/solver/learn/IExplanation.java	0% <0%> (-100%)	0% <0%> (-1%)
...er/constraints/nary/binPacking/PropLoadToItem.java	0% <0%> (-100%)	0% <0%> (-17%)
... and 337 more

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