Remove dependency of FunctionSpaceElement on Operator

[kohr-h]

This was a lazy way of implementing function spaces in the first space. Probably the usefulness of all the Operator stuff is marginal. Removing this dependency would greatly improve the flexibility of vectorized functions, e.g.

>>> def f(x, c):
...     return sum(x) + c
...
>>> fspace = odl.FunctionSpace(odl.IntervalProd([0, 0], [1, 1]))
>>> f_elem = fspace.element(f)
>>> points = np.array([[0, 0], [0.5, 0], [1, 1]]).T
>>> f_elem(points, c=0)
array([ 0. ,  0.5,  2. ])
>>> f_elem(points, c=2)
array([ 2. ,  2.5,  4. ])

This is not possible currently because Operator has a very harsh signature restriction that is not needed here.

[adler-j]

Fine by me!

[kohr-h]

Closed by #1088