Renamed file and example impact function
Updated function and output slightly
Updated example code from running impact function
First cut of impact function guide done (#487) - still need example with assign_hazard_level_to_exposure.
Automatic updates of api-docs
Small fix in impact function docs
Moved docstrings from Vector and Raster constructors to the class level docstring. The reason is that the automatically generated documentation ignores private methods such as the constructor.
Started on a generic description of impact function development
Resolved merge conflicts from master