Refactor special_particles.py
Refactor special_particles.py

 - Put special attributes into a dict that is then combined with the
   existing attributes dict for each particle.
 - Rename variable `thing` to `particle`.  There is potential for
   ambiguity about whether something is a particle or antiparticle,
   but `thing` is perhaps not the most descriptive word.

StanczakDominik/PlasmaPy