Replaced `waLBerla_python_file_generates` with `waLBerla_generate_target_from_python`

This new function does the actual generation using pystencils and creates a target library that can be used as a dependency as usual. One should not list the python script itself anymore in add_executable.
Convention: Use the AppName as a prefix for the generated lib!

Example Usage:

waLBerla_generate_target_from_python( NAME AppNameGeneratedLib FILE GenerateKernel.py OUT_FILES Kernel.h Kernel.cpp)

waLBerla_add_executable ( NAME AppNameGeneratedLibTest DEPENDS GeneratedLib)