Add 'SubdomainCompoundField' and 'InterfaceCompoundField'

examples/helmholtz/crossPoints/README.md

examples/helmholtz/crossPoints/mesh.cpp

src/field/CMakeLists.txt

src/field/InterfaceCompoundField.cpp

src/field/InterfaceCompoundField.h

src/field/InterfaceField.cpp

src/field/InterfaceField.h

src/field/InterfaceFieldInterface.cpp

src/field/InterfaceFieldInterface.h

src/field/SubdomainCompoundField.cpp

src/field/SubdomainCompoundField.h

src/field/SubdomainField.h