Remove inverse Compton and pion decay emission code.

This is now available in gammafit.
Add PositionDependentMultiGaussPSF class
Begins to implement iterative background estimation algorithm - still in progress
Completes addition of basic code - still unit tests and examples to add
Begins to add unit tests
Merge branch 'iterative_background_estimator' of github.com:ellisowen/gammapy into iterative_background_estimator