reader-centric bells and whistles

[bueler]

Main thing: Please ignore this if it is not to your taste.

I am reading your delightful book and running all the examples. I fiddled in the following ways on the Chapters 3 and 4 examples (so far), because I think it makes it clearer to readers how they work and what they do:

• fixed the use of plot(), which breaks in current Firedrake, in ch3 codes, to use tripcolor() or quiver() as appropriate
• add a leading line indicating which (sub)section the code corresponds to, and which figures
• since I am running codes in parallel in ch4, changed the printing to use PETSc.Sys.Print()
• added visualization in {2|3}d_product_elements.py
• added printing of t in days in sw_linear_w2.py and sw_nonlinear_w5.py
• added .gitignores and makefiles, the latter just for a make clean target to clean up results and figures

I'll do the same fiddles in ch5_examples/ pretty soon, so no rush to merge.

Only the first item is a bug fix. If you just want that as a pull request, let me know.