Add new file and wrapper class particle_decays

It will be necessary to look at the decays of multiple particles at the
same time and therefore we to access this process in a class
Add the option to give the ref_file a custom name
Add translation for phi listing
Add functionality to actually extract BR for decay
Add first test for particle_decays class
Add parameter for filename to extract_decays
Add function to read evtgen source file - untested
add doc-files for particle.py and test_particle.py