Improve Burgers solver example

[rouson]

• Set time step based on a stability criterion and safety factor
• Loop until finish time
• Verify result
  • Compare to the exact solution in Fig. 4.6 of Rouson, Xia & Xu (2011)
  • Check shape based on fixed points and concavity
• Ensure tests pass with all supported compilers
  • LLVM flang
  • Intel ifx
  • LFortran lfortran
  • GCC gfortran
  • NAG nagfor