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 think the wording "direction cosine matrix" is not that common. And "dcm" isn't that informative. For this general vector package, would it be better to call it `rotation_matrix` or `rot_mat`.