Some edits to chapters 7, 10

[rjleveque]

Some changes:

Nonconvex_scalar:

• Removed the section on Linear Degeneracy as out of place (and expanded the discussion of this in Euler and Shallow_tracer)
• Improved the discussion some place, in particular of convex hull

Euler:

• Added titles to some plots generated in exact_solvers codes, and changed interactive Riemann solver for Euler so it has different data than Sod, and so it's clear what these data are in the call to euler_demo1, to improve what shows up in the pdf file.
• Changed lots of bold text to italics to better match what we do in other chapters, where bold is rarely used.
• Reorganized definitions of genuinely nonlinear and linearly degenerate to discuss more in more generality.
• Added a bit of discussion to the last two sections on vacuum states.

One question in Euler: do we really want to write out the full 3D Euler equations, as we do in this notebook? We don't do this for any other problem, although we could for acoustics, SWE, etc. Maybe it's useful to see but perhaps just distracting since we don't discuss it further.

[ketch]

These changes look good to me. I think we should remove the 3D Euler equations, since we don't even talk about shear waves in the Riemann solution, and interested readers can go to the other books we cite.

[rjleveque]

I removed 3D Euler and slightly reworked the paragraph around the 1D equations, and eliminated one section heading.