[Review]: Test map sources (#664)

I'll be adding tests to classes in map/sources in this PR (issue #664 ). What I have now is by no means the final result, and more stuff will be added in due time.
This is going to be one long PR!
@cadair Sorry, was out.  I was referring to the numpy testing functions: http://docs.scipy.org/doc/numpy/reference/routines.testing.html