Tidy up extra directory and decide whether to keep presentations there

[matt-graham]

We now have scripts for doing scaling runs on various different systems scattered over a few branches

https://github.com/Team-RADDISH/ParticleDA.jl/tree/mmg/kathleen-scaling-runs
https://github.com/Team-RADDISH/ParticleDA.jl/tree/scaling_archer2
https://github.com/Team-RADDISH/ParticleDA.jl/tree/tk/thread_pinning

Ideally we should probably have all the cluster specific job scripts we have been using for scaling runs on the master branch along with a shared Julia script used in all of them.

I also have the Pluto notebook I used for recent presentation on a branch on a fork at

https://github.com/matt-graham/ParticleDA.jl/tree/mmg/crism-presentation/extra/crism-fsds-workshop-presentation

and Serge has previously shared the LaTeX source for a presentation he gave.

Not sure if we want these to live in the main repository in extra directory as well, but we probably want to have some system of keeping track of them centrally to make it easier to reuse slides.