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 
Each coordinate system needs a del operator of its own. A del operator differentiates wrt the coordinate variables, which are unique to each coordinate system. Hence the assignment of a unique Del operator to each.

About the circular dependency issues, I have tried many designs to get rid of them. But because the concepts are interrelated, I don't think we can avoid a few of them. For example, vector.py needs code from coordsysrect.py, which needs code from deloperator.py, which in turn needs code from vector.py. Its either this, or putting it all into one file, which I am not comfortable with.