fix uservars in minizinc

[Wout4]

All solvervars were being used as uservars in minizinc, from a time before we needed decompositions for minizinc.

[Wout4]

Not sure if we need to uncomment the get_variables in objective() (line 401)
If there can be vars in the objective that are not in the constraints we need to?

[IgnaceBleukx]

Seems good, we indeed need to keep the line adding unknown variables in occuring in the objective

[Wout4]

keep it? it's not there at the moment i think

[Wout4]

Seems like putting variable in the objective that is not in the constraints doesn't work anyway

m = Model(x1 == 6)
m.minimize(x2)
m.solve('minizinc')

x2 will be None after solve even if we add it to the uservars.
Merging this