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)
That's fine. But please keep following the pattern where you have the actual code in the `.cross()` method (for example) and then overload the operators with by calling cross. In the current physics vector code, the actual code is under `__xor__()` and I don't find that very clear and intuitive. I think my main issue with this is rooted in the fact that Python is not a language written for mathematics, it is a general language in which we do mathematics. By default, we don't get to have all the various mathematical operators available so that the written code looks like a math text book and I don't think we should try to force it to. There are a limited number of operators in Python and they have default meanings. If every module written in Python overloaded them to mean different things, then it becomes infinitely difficult to read Python code. I think it is better to stick to a something like "if we are doing something like addition, then it is probably ok to overload the + operator" and this would go the same for "if we are doing something like bitwise exclusive or then it is ok to overload ^". Maybe this is a good read: http://stackoverflow.com/questions/1552260/rules-of-thumb-for-when-to-use-operator-overloading-in-python, I'm sure there are others.

My only preference is that we default to the custom named methods and use that by default in the code so it is easy to read without having to remember how you've changed operator behavior. I don't think "mathematical notation with python code" is the priority. If you overload these odd operators, then this should be demonstrated as an option not the norm.