changes to episode 6, tasks 1-9

[mallewellyn]

Largely re-writing motivation and methodology sections in line with tasks 1-9 of #119

Particularly tries to clarify the differences between clustering and PCA/FA from the outset (even though they're completely different methods, I think a reader would benefit from this being apparent immediately).

Also moves the section on believing in clusters to after the method is explained - I think how to believe in a cluster is easier to understand if we know what clusters are fully first

[mallewellyn]

All fixed above :)

[alanocallaghan]

Awesome thanks, great addition