1.6.0

• Improved GenMultiDecay to have better control on the decay mass of non-stable particles.
• Added a particle_model_map argument to the GenMultiDecay class. This is a
  dict where the key is a particle name and the value is a mass function name.
  The feature can be seen in the
  GenMultiDecay Tutorial <https://github.com/zfit/phasespace/blob/master/docs/GenMultiDecay_Tutorial.ipynb>_.