Handle function default arguments better

[Martin Sjölund]

We need to handle function default arguments better. Either by saying we elaborate into f(x,$defaultValue) and creating special C-code that handles this... Or by adding the correct expression in the DAE.Exp.

function f
  input Real x;
  input Real y := x;
  output Real s := x+y;
end f;

model M
  Real x = f(y);
  Real y,z;
end M;

[Per Östlund]

Fixed in r19848.