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
Just a personal preference here. I'm not a fan of the short operator notation as I feel it impedes readability. Not saying we shouldn't have it, but a docstring about `dot` should start with a `dot` example, and then say something about "operator notation is also supported using hte `&` operator:", then an example using the operator.