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)
On the other hand, it would also be nice to have a unique `CoordSystem` object throughout sympy. Unfortunately it would be harder to maintain because of the cross links that this approach entails, but this would avoid code rewriting.