PyOP2 classes are not what you thought they were.

I find myself, when I'm in firedrake land, writing things like the following.

````python
def is_set(x):
    # Is x a Set?
    return isinstance(x, op2.Set)
````

Try it:

````python
In [1]: from pyop2 import op2

In [2]: op2.init()

In [3]: s = op2.Set(10)

In [4]: isinstance(s, op2.Set)
Out[4]: False
````
huh?