Improvements on flow control accessors
New feature: the ChRandomShapeCreatorFromFamilies can define the percentuals of the families in terms of created mass, in alternative to the default percentuals on number of created particles.
This is especially useful because in many cases, granulometry data is provided in terms of masses, for particle types, and this is not easily related to the amount of particles, especially if each family has different distributions for random sizes, shapes, etc.
Fix a potential bug with iterators. If the particle event is of type ChParticleProcessEventRemove, the iterator in the previous implementation would be invalidated during the iteration, and might create random segfaults.
Improved emitter demos.