(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 
I believe a rectangular coordinate system is another name for the cartesian system. Each 'type' of system is named by the kind of geometry it describes - spherical, cylindrical, rectangular.
Some resources on the web call it that way too-
http://www.wtamu.edu/academic/anns/mps/math/mathlab/beg_algebra/beg_alg_tut20_rect.htm

So, if we in the future have a class like <code>CoordSysSpherical</code>, going with the current name seems more intuitive to me.