Updated the calculation of findnearestneighbour()
Updated get_ir to work with 3D HRTFs
Updated the handling of the WFS pre-filter

Now every pre-filter should be directly applied by wfs_preequalization(). In
this function we will integrate the handling of different filters and the
analysis of conf.wfs.usehpre
Simplified calculation of secondary_source_distance
Updated use of WFS pre-filter function
Tried to fix the amplitude in imp movies

As a first workaround we multiply the wave field p by 2. But this should be done
in a better way
Updated all SFS_binaural_synthesis function to 3D and new secondary_sources