draft reorganization of helmholtz examples

contrib/bessel/CMakeLists.txt

examples/helmholtz2d/diskScattering/CMakeLists.txt

examples/helmholtz2d/diskScattering/README.md

examples/helmholtz2d/diskScattering/main.cpp

examples/helmholtz2d/waveguide/Readme.md

examples/helmholtz2d/waveguide/longitudinal/CMakeLists.txt

examples/helmholtz2d/waveguide/longitudinal/main.cpp

examples/helmholtz2d/waveguide/longitudinal/main_length.cpp

examples/helmholtz2d/waveguide/longitudinal/main_omega.cpp

examples/helmholtz2d/waveguide/transverse/CMakeLists.txt

examples/helmholtz2d/waveguide/transverse/main.cpp

examples/helmholtz2d/waveguide/transverse/main_length.cpp

examples/helmholtz2d/waveguide/transverse/main_omega.cpp

examples/helmholtzFlow/flowPastCylinder/CMakeLists.txt

examples/helmholtzFlow/flowPastCylinder/main.cpp

examples/helmholtzFlow/freefield/CMakeLists.txt

examples/helmholtzFlow/freefield/main.cpp

examples/helmholtzFlow/freefield/runTests_circle.sh

examples/helmholtzFlow/freefield/runTests_square.sh

examples/helmholtzFlow/helmholtzDuctHeteroX/CMakeLists.txt