Abstract constraints & system

[gchabert]

For different extensions of ibex (like SIC solving, SIP, min-max optimization or integral-based constraints/objective, etc.), we need a more general structure than the current System.

A solution might be to create an abstract class Ctr for constraints, with definition of services like the default associated contractor and self-relaxation/restriction; and an abstract class Sys for systems, which would contain a Fnc object as objective (this interface already exists) and a set of Ctr constraints.

This way, many algorithms would have a wider scope.