add whiskers and vectors objects for plotting

[beckermr]

This one is pretty self explanatory. The question here is about what the interface should be. There are a few options.

1. Accept points and a vector like biggles.Whiskers(xloc,yloc,vec) where vec is a (N,2) numpy array or maybe list of tuples/lists.
2. Accept points and vector components like biggles.Whiskers(xloc,yloc,u,v) where u and v are the x and y components of a vector.
3. Accept points and Q,U or e1,e2 spin-2 fields, biggles.Whiskers(xloc,yloc,Q,U). This last interface is much more astrophysics domain specific, so might not be the best idea.

We can easily make a biggles.Vectors object at the same time that is called like this biggles.Vectors(xloc,yloc,u,v) and plots a vector at location (xloc,yloc) pointing to (u,v). The trick here is specifying the shape of the head of the vector. Simple lines would be easy. A filled triangle head would be a bit more difficult.