[examples] Use BoundaryTypes directly

examples/1protationsymmetry/doc/problem.md

examples/1protationsymmetry/problem.hh

examples/1ptracer/doc/1p.md

examples/1ptracer/doc/tracer.md

examples/1ptracer/problem_1p.hh

examples/1ptracer/problem_tracer.hh

examples/2pinfiltration/doc/2p.md

examples/2pinfiltration/problem.hh

examples/biomineralization/doc/setup.md

examples/biomineralization/problem.hh

examples/freeflowchannel/README.md

examples/freeflowchannel/problem.hh

examples/shallowwaterfriction/doc/swe.md

examples/shallowwaterfriction/problem.hh