Make NEI.simulate() not return results but rather store results within the class instance?

[namurphy]

Comments from code review in #20:

I wouldn't have simulate() return results at all, since that encourages users to separate the results from the originating simulation. I would encourage the use of...

>>> sim.simulate()
>>> sim.results.plot_something()

Or, if the result was a xarray.Dataset then one could do...

>>> sim.simulate()
>>> sim.results.T_e.plot()

[StanczakDominik]

I'm not sure I agree with the review comment there; I think I like having the simulation object be sort of a set of initial conditions, and then simulate could return an xarray.Dataset of results while allowing one to vary, say, the timestep or other non-physical simulationy parameters. But that's just one approach; I could definitely be convinced otherwise :)