Feature/language affectation rework

This means that now the following code is valid
```
import scheme;
import mesh;
import math;

let m:mesh, m = cartesian1dMesh(-1,1,100);

let pi:R, pi = acos(-1);

let f: R^1 -> R, x -> x[0]*x[0];
let g: R^1 -> R, x -> sin(pi*x[0]);

let (fh,gh): Vh*Vh, (fh,gh) = (interpolate(m, P0(), f), interpolate(m, P0(), g));
```
This allows to define builtin functions that return composed
results. For instance the result of the calculation of a time step of
Lagrangian schemes. One can now return (rho,u,E,...).

Now the left- and right-hand-side are given explicitly to
getNodeProcessor function and thus to the processor constructors. The
aim of this change is to break the dependency on the affectation node
itself and thus to allow their use in list affectations.