(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 
Is this simplification always only acting on expressions of a single rotation matrix multiplication? i.e. is is always R1 * R2? where the expressions in R1 and R2 are only as complex as the rotation method (euler, quaternions, etc)? If so, I suspect you can pick the best method based on simplification and compute time, and maybe it is worth ok to not even select the method explicitly, letting trigsimp choose the best.