Basic vector code, without a coordinate system class

All calculations essentially occur in the same frame.
i, j, k and x, y, z are imported as base vectors and base scalars by default on importing sympy.vector.
delop is a Del operator instance.

Example:

>>> v = 3*i - j
>>> delop ^ v
0

(delop ^ v represents curl of v)
I think that `__eq__` tests first the type equivalence, then the hashes. The hash is cached once it has been computed the first time. Your tuple comparison would recompute the hashes of its objects every time. But I'm not very sure about this. Maybe @asmeurer can help.