provide raster processing functions as class functions

The current way of registering new functions that are exposed by the backend module is via a python class. Currently the processing function is not part of a class, so I can't register it at the backend module. Therefore I will put a class wrapper around the function to make it work.

The latest backend api (v0.1.x) supports the registration of classes and standalone functions. So we might want to switch to the latest version sometime soon.