Full support for physical entities. Mandatory with MPI.

doc/lua_api.md

include/libgmshdg/gmsh_io.h

include/maxwell/maxwell_common.h

share/validation/params_twomat.lua

share/validation/twomat.geo

src/libgmshdg/gmsh_io.cpp

src/libgmshdg/param_loader.cpp

src/maxwell/maxwell_common.cpp

src/maxwell/maxwell_cpu.cpp

src/maxwell/maxwell_gpu.cpp

src/maxwell/maxwell_interface.cpp

src/maxwell/maxwell_postpro.cpp

src/maxwell/maxwell_solver.cpp