LinearIOSystems cannot be multiplied by a scalar/array #541

The following code causes a bug when it tries to raise an error:

import control as ct
import control.iosys as io
2*io.ss2io(ct.ss(1,1,1,1))

raise NotImplemented("Scalar multiplication not yet implemented")

This is because InputOutputSystems define various methods, e.g. __mul__ that override those in StateSpace and some are not implemented yet.

Options that come to mind if we want this to work better:

1. Test for instances of StateSpace correctly and dispatch. This seems to be onerous and at odds with how inheritance ought to make things easier.
2. In the __init__ method of LinearIOSystem, delete those methods
3. move those operations to NonLinearIOSystem (and override as necessary in LinearIOSystem when it makes sense to create a new LinearICSystem)

Related: many of the methods like __mul__ don't act on self and other but rather sys1 and sys2. This might still work, but I think the first argument should be self according to Python convention.