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)
Uhm, maybe that it should no longer inherit from `Dummy`? I mean, `Dummy` is a symbol whose instances are not equal (`Dummy("x") != Dummy("x")` vs `Symbol("x") == Symbol("x")`), that makes sense in case the `name` is used many times for instances which are not mathematically equivalent. But if you choose the `args` in order to uniquely identify the instance, you don't want to inherit `Dummy`. Maybe inheriting `Basic` or `Expr` is a better solution.