Implement minimization through constrained algorithm.

[corettialessandro]

Implement the function Constrained() to perform constrained minimization using the Lagrange multipliers theory.

[corettialessandro]

Done, additional testing is necessary even if preliminary tests have given good results.

It will be good to unify the three functions Unconstrained, Constrained and Reduced on the basis of the relation btw N_var and N_constr

[corettialessandro]

Implement value of constraints to enforce in the unconstrained case from input files.
Implement possibility of enforcing two different partial sums on different sets of variables.