Add a different nbody simulation

[WhoisDavid]

• add an n-body simulation of the solar system as 'alternative' n-body
• fix a typo from previous commit which moved the alternative game of life version.

[cart]

I think I'd prefer to just allow different implementations of the same thing, rather than categorize the "thing" that came second as "alternate". Would you mind adjusting this to just add a new line like normal (and maybe not right next to the other impl to avoid confusion)?

[WhoisDavid]

Yeah sure thing! I just appended mind then.
I was also considering splitting Simulations from Games FYI but unclear if this is relevant at this stage.

Re: Game of Life, I left it as it was before (with alternative) because in this case one is actually a fork of the other.

[cart]

Yeah I think at this point we don't need to distinguish them. Once we migrate to a more interactive awesome-bevy I think it makes sense to start adding more categories.