SOLPS rewrite and tidy-up

[CnlPepper]

There is a lot of bad code in this package that really needs addressing. At present the SOPLS sim object has no way of being initialised without modifying internal, hidden attributes. I've spotted a few cases of API inconsistency and incorrect docstrings.

[mattngc]

To be honest, we need a new maintainer for this module. I might post it somewhere. I wrote this module 3+ years ago and have barely worked on it since. Ideally we should find a SOLPS developer or heavy user to look after it.

[vsnever]

I think SOLPSSimulation class should be initialised with mesh, species_list instead of just mesh, because this will allow to define property setters that check array dimensions.

[vsnever]

Is is safe to remove parallel_velocities and radial_velocities attributes from SOLPSSimulation? Seems like duplicates to velocities_parallel and velocities_radial.