Remove inverse Compton and pion decay emission code.

This is now available in gammafit.
Add PositionDependentMultiGaussPSF class
Begins implementation of Lafferty and Wyatt flux point method and API changes to flux_point.py

Adds xfail test command to added unit test
Add GaussianBand2D model
Add GaussianBand2D to docs
Improves flux_point API, adds lafferty method and unit tests and adds tutorial example
Makes some changes to the flux point tutorial to correct errors and improve clarity
Merge pull request #128 from ellisowen/flux_point_method

Flux point method