Improved the error message for unavailable gsims

If the user mispells the GSIM class name the error message displays the list of available classes. 
Fixed some PEP8 issues and renamed one test. Notice that by construction
get_available_gsims cannot return classes which are not subclasses of GroundShakingIntensityModel. See also https://github.com/gem/oq-hazardlib/pull/109