slightly improved report-pre; add TODO for Jacobian

I feel like it's necessary to introduce Jacobians.
wrote the model section

For introducing the models we are wrestling with (C_1(N), C_1(N),
explicit equations of them, etc.), and results in the literature.
move old code out of report.tex

The computational part is now in report-comp.tex, waiting for a clean up
and rewrite.

Robin's previous work is moved to report-staging.tex, since I've no idea
how to integrate them yet. We will figure this out after some
discussion.