(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 
What's the reason for the simplify flag? I think things are more flexible and less to maintain if we just let the user use whatever simplification they want to use.