Plasma documentation.

[aoifeboyle]

@unoebauer @wkerzendorf @chvogl @yeganer

Dug out the old plasma documentation I forgot to merge. It's not loads, but it's a good outline. I will add a how-to at the end; let me know if you have any other suggestions.

[unoebauer]

@aoifeboyle Very good - this is an ideal starting point for a detailed documentation of the plasma module. Having a little how-to section at the end would certainly be very helpful.

[wkerzendorf]

@unoebauer - can you link this PR in your PR. We probably want to include this as part of our documentation project.

[unoebauer]

I think we should finally merge this, since it gives a more up to date rough overview of the plasma calculation even though a lot of information is still missing

[unoebauer]

We close this - the relevant bits of the documentation will be updated as part of a later PR