Don't generate mesh if input is .msh.

include/libgmshdg/gmsh_io.h

share/examples/capacitor2/capacitor.geo

share/examples/capacitor2/params_capacitor.lua

src/libgmshdg/gmsh_io.cpp

src/maxwell/maxwell_solver.cpp

tests/test_basics.cpp

tests/test_differentiation.cpp

tests/test_differentiation_gpu.cpp

tests/test_lifting.cpp

tests/test_lifting_gpu.cpp

tests/test_mass.cpp