Applying BBS observer effects to composition

[ethanwhite]

Since observer effects influence species richness values in BBS, we need to discuss whether we need to apply observer models to the composition data as well. The observer effect will likely only create changepoints in composition if abundant species are being impacted. Need to discuss:

• [ ] are richness changes mainly in rare species?
• [ ] how do rare species influence LDA groupings?
• [ ] do we need to model out observer effects on abundances, if so, how?

(Moves over @skmorgane's issues from https://github.com/weecology/macro-ts-struct/issues/2)

[ethanwhite]

The value of this depends on whether or not the first analysis is focused on BBS or more broadly. In a lot of other datasets we may not have observer ID. That said, being able to do this comparison using BBS might still be valuable as a way of providing some context for how important observer changes might be in other data sources.