for PETSc v3.13.2

C codes (Chapters 1--12) were tested with PETSc master branch at v3.13.2.

Python codes (Chapters 13--14) were tested with Firedrake downloaded 18 June 2020.

Gmsh version 4.5.6 was used in testing Chapter 10, but with -format msh22 to generate legacy (version 2.2) format. The default format is 4.1 now. When issue #46 is fixed, it should not matter which gmsh is used, and the -format option should be unnecessary.

Test machine: bueler-thelio ubuntu 20.04