added annex figures, tables, equations

[de-code]

resolves #737

[coveralls]

Coverage increased (+0.006%) to 38.943% when pulling 6f45c06 on elifesciences:back-section-figure-tables-upstream into 4073084 on kermitt2:master.

[de-code]

I am not quite sure how I managed to increase the coverage without adding tests or deleting code.

[lfoppiano]

With the standard model, I cannot see any difference, the "Extended Data Figure 1" is not extracted at all in both cases.
I've checked the code and the PR looks good to me.

[de-code]

I noticed one issue with it, is that the figure ids are reset. e.g. first body figure has the id fig_0. Then the first back section / annex figure also has the id fig_0.