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)
Doesn't this do the same thing?

```python
vec = Vector.Zero
for k, v in self.components:
    vec += diff(v, *args, **kwargs) * k
return vec
```

You have this kind of code several times. Maybe this would make things more concise and readable.