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)
I have initialized them here as default instances.
<code>
from sympy.vector import i, j, k
v = 3*i + 4*j + 5*k
</code>

After CoordSysRect,
<code>
C = CoordSysRect('C')
v = 3*C.i + 4*C.j + 5*C.k
</code>