Supported particles

The following particles are currently supported:

particle name	pdg_code	rest_mass	charge
electrons	11	0.511 MeV	-1.602-19 C
positrons	-11	0.511 MeV	1.602-19 C
gammas	22	0 MeV	0 C
protons	2212	938.272 MeV	1.602-19 C
neutrons	2112	939.565 MeV	0 C
optical_photons	0	0 MeV	0 C

Adding new particles

Adding new particles is simple:p

update the particle_properties dictionary in ParticlePhaseSpace.__particle_config__.py:

particle_properties = {
    'some_new_particle':
    	{'rest_mass': rest_mass_in_MeV/C^2,
         'charge': charge_in_coulombs,
         'pdg_code': pdg_code}
}

Below the dictionary definition, add an alias for the particle. This enables various parts of the code to operate using either particle names or pdg_codes:
```
particle_properties[pdg_code_new_particle] = particle_properties['some_new_particle']
```
Update the documents above with the new particle so the tests pass