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
The SymPy core needs to make sure that the args of every instance of any subclass of Basic, are _all_ instances of some Basic subclass, on their own.
So these tests instantiate an instance of these subclasses, and then check is all of its args are
<code>isinstance(Basic)</code>