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.
I would suggest to do is instance check. This `particle` object might implement the particle api but not be an instance of the python Particle class but a c python extension class. if the api is not right then remove_element will raise as appropriate.