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.
In fact with that check we don't need to put the default value as `particles=None`; if the user passes a wrong particles sequence or a `None` value by accident it will fail anyway raising an Exception. And if the user doesn't specify a particles sequence python will raise a TypeError exception, so yeah, I'll remove the default value.