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 
This is true, but not a very informative explanation. Space fixed rotations are such that the rotations are always about the base frame's axes. Body fixed (euler angles) are such that the rotations are always about axes fixed to the body being rotated.