(WIP) Basic Vector framework

Added:

1. Vector core - Vector, VectorAdd, VectorMul, BaseVector
2. BaseScalar
3. Del
4. CoordSysRect
5. Point
6. functions.py - express, matrix_to_vector 
I still think it should be called Cartesian, though. While some people might refer to it as a rectangular coordinate system, it's not that common.