Support custom atomic operators

[jakobj]

This PR introduces a decorator which users can apply to custom functions which are then available as atomic operators in the strings defining the operation of custom nodes. Hard to explain, easy to show:

    @cgp.atomic_operator
    def f_scale(x):
        return 2.0 * x

    class MyScaledAdd(cgp.node.OperatorNode):
        _arity = 2
        _def_output = "f_scale((x_0 + x_1))"

These few lines introduced a new primitive, i.e., a new node function, to CGP. This approach also works for custom atomic operators which depend on external libraries.

closes #235

[coveralls]

Coverage increased (+0.02%) to 94.466% when pulling 54d1baa on jakobj:enh/custom-atomic-operators into f6143c5 on Happy-Algorithms-League:master.

[HenrikMettler]

This is so nice 😍 Maybe I like it too much to be critical, but everything looks good to me

[mschmidt87]

Beautiful.