Add particle code

[mirenradia]

I'm not sure if this is supposed to be just to Chombo or also for GRChombo but I guess you'll have to add it here first anyway.

[cjoana]

In principle Chombo has already a particle code implemented. So probably we just need to adapt GRChombo to use it.

[mirenradia]

I think the code was removed from the current version of Chombo but @KAClough emailed the Chombo developers and they sent it to her.

[KAClough]

In case I get hit by a bus I should probably note that you can find this here:
https://github.com/KAClough/Chombo_3.2/tree/withParticleTools