Minor edits

[jeffreypullin]

Hi Kate,

This PR contains some small cleanup/refactoring of the general repo (first two commits) and the analysis of two component mixtures (last commit).

The changes to the analysis are (intended to be) non-functional. They just represent me:

• Improving the density evaluation code (index, don't eval 😄)
• Removing some of the 'on the journey' plotting code and bringing it into line with personal ggplot2 style guide
• Finessing some of the rmarkdown details

Overall though, I think your code looks great - feel free to apply (or not) these changes to other sections if you want.

Best, Jeffrey