Make (scalar) MX constants print as numbers

Currently, constants in MX expressions print as e.g. `Const<5>(scalar)`:

    In [1]: from casadi import *

    In [2]: 5*MX('x')
    Out[2]: MX((Const<5>(scalar)*x))

I want to request that constants be printed just as literals instead, e.g. `5`. The current format is scaring our users.

This patch changes printing of scalar MX constants, which would go a long way for us. Whether you want to take the patch or not, I hope that you can take this as a request to simplify printing of constants in MX expressions.