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)
One use case for the unevaluated expressions is easy mapping to code in the code generators. For example, Integral can map to scipy.integrate.trap. Del could map to numerical gradient functions. I'm not sure we have to figure out the use case, but it just seems like we are introducing an operator that does differentiation of sorts, but are not following the sympy pattern for these things, which is to have both an unevaluated form and a function that creates the form and evaluates it.