Main chris edits

[cmcooling]

I've made a number of changes here. These were mostly minor, but a more significant one was that I switched the notation in SPH.md to use bold variable names for vectors. I think I've got this right, but it's worth checking.

Please check through the changes and let me know if you have any questions. It's possible I've misunderstood something so please don't accept blindly and tell me if you think I'm wrong!

[ge420]

@cmcooling Some of the changes that you made in the SPH.md are not displayed properly in the repo. For example instead of seeing the resulting equations I see them in LaTex form. Is it only me?

[cmcooling]

Yuck, this is something horrible about GitHub Markdown. Sometimes it just doesn't render in markdown for some reason. It works fine in VS Code. It also works in several cases if I use $[expression]$, or if I use ```math to annotate it. But some of these then break it in VS Code. I'm not sure exactly which markdown flavour will make the final website, so we need it to be as portable as possible.

I'll see if I can find a way to make this render nicely in every occasion when I have a minute.

[cmcooling]

@ge420 I found the problem in the Markdown - a set of curly brackets followed by an underscore, followed by another set of curly brackets breaks mathmode. It's a known bug. The workaround is to leave a space after the underscore. This seems to cause it to render correctly in GitHub and VS Code.

So I think it should be ok now, but please feel free to check the formatting and the content before merging.