Improvements to the vector module: Del and Orienters

1. Introduces the Orienters API for good code structure
2. Del class now returns unevaluated expressions(vectorial or scalar) composed of Derivative instances. The user can call .doit() to evaluate them.
3. adds the following printing implementations for the vector module - pretty, latex
4. scalar\_map method now takes into account the relative location of one coordinate system wrt another.
5. adds the methods 'measure\_numbers()' to Vector
6. Adds helper methods for calculating curl, divergence, gradient. Also adds the scalar\_potential and scalar\_potential\_difference functions present in the physics.vector module
I want to discourage the user from instantiating coordinate system, except for the 'root' of the tree. Otherwise there is a dedicated method to do everything else. The constructor needs to just accept just a rotation matrix, since all the matrix-multiplication and orientation code is with the Orienters and orient\_new.