Fix latex syntax errors in .plot files in analyses

I hopefully fixed most of the LaTeX syntax errors in labels in the .plot files in the analyses directory. Note that this is "just" LaTeX errors; the labels have not been made compatible with matplotlib mathtext yet. However, I thought these changes could be merged beforehand.

I am a bit unsure whether the change I made in analyses/pluginMC/MC_MET.plot is correct, so I would appreciate if you could check those changes extra carefully.

Since I have not contributed to Rivet before and am quite new to open source in general, I might have made some mistakes, e.g., how many approvals should be required, which labels that should be added to the merge request etc. Any feedback on this is appreciated!