conditional defined for SX, but expand MX conditional fails

[ghorn]

import casadi as C

x = C.SX.sym('x')
y = C.conditional(x, [x], x)
f = C.Function('lolwut', [x], [y])  # works, SXFunction


x = C.MX.sym('x')
y = C.conditional(x, [x], x)
f = C.Function('lolwut', [x], [y])  # works, MXFunction
f.expand() # fails

[jaeandersson]

Missing feature

[jaeandersson]

Missing feature

I did say missing feature, but it's sort of weird to have the expanded version do something else completely...

[jgillis]

Can be compacted after #1897

[jaeandersson]

Made some more commits on #1897, related to Function::operator() and Function::call. These functions had already been made const internally so updating the public functions were trivial.