Two important changes : a bug and Supernova monte carlo aproach

1. A bug in starformation.c When new particles are created and when they are append to the end of the list, be cause no particles between start and newstars reside in the core (typically no dark matter), the new particle was found without a correct correspondance. This is due to a case not treated in the reorganisation of the particles.

2. Add the possibility to form only entire stars, using a monte carlo aproach. This modification is done in chimie.c