Feature particles

Hi everyone,
this pull request is to integrate the implementation of the stand-alone clases of the particles group.
There are currently four files:
    - abstractparticles.py: where the ABC is defined and some common parts of the clases.
    - particlesclasses.py: here is the implementation of the ParticleContainer, Particle and Bond classes.
    - pcexceptions.py: here I defined some custom exceptions to make the code clearer, I though it was a good   idea to keep them centralized in a file.
    - test_particlesclasses.py: unit tests of all the clases.

Thanks for reviewing it and any comments, suggestions and improves would be appreciatted.
While they exist obfuscated private methods are very rare in python code. I would suggest to use a single `_` instead. Furthermore, using `__<name__` is mainly used for python special magic methods.