Double check protected data member

[claudiofantacci]

Many classes have way too much protected data member that should probably be moved in private scope. See for example GaussianMixture and derived classes.

cc @xEnVrE

[xEnVrE]

See this for reference.

[claudiofantacci]

The current library rework is addressing this.
The main problem seems related to GaussianMixture and derived classes where we need to carefully think about the consequences of this change.

[claudiofantacci]

To date, if you change the scope of

unsigned int num_particle_;
std::size_t state_size_;
ParticleSet pred_particle_;
ParticleSet cor_particle_;

in SIS.h it becomes difficult to inherit from it (obviously, we don't have proper interfaces to those data). As a matter of fact, the user cannot access that data at all and this is no longer acceptable as they are the product of the class itself. I'm going to change this behavior.

@xEnVrE we need to discuss GaussianMixture f2f asap.