Examples dir is bitrotting

More than half of the files in examples/ do not work when run:

• cahn-hilliard.py
• fmm-error.py
• maxwell_sphere.py
• two-domain-helmholtz.py
• find-photonic-mode.py
• perf-model.py
• find-photonic-mode-sk.py
• helmholtz-expression-tree.py
• maxwell.py
• stokes-2d-interior.py (works but is Py2 only)

In addition, the examples that do work give a number of warnings such as "Executing global QBX without refinement".