Fix reification of constraints with optional variables

[arbimo]

Expressions with optional variables should be reified to optional literals

For instance a constraint a < b should be reified to a literal l such that

• prez(l) <=> prez(a) && prez(b)
• l = true iff prez(a) && prez(b) & int(a) < int(b)
• l = false iff prez(a) && prez(b) & int(a) >= int(b)
• l = absent iff !prez(a) || !prez(b)

Currently the value of l is unspecified if one of the variables is absent which is unsatisfactory and requires the addition of specialized constraints such as OptLeq