Support for variable functions, i.e. callbacks

[glenfletcher]

Make the following work

from Equation import Expression
import numpy as np
fn = Expression("(y(x+(d/2))-y(x-(d/2)))/d")
y = lambda x: x**2
x = np.linspace(0,2,1000)
d = 0.0005
dy = fn(y,x,d)