Improved SILO field export. Nodal and Zonal exports now possible.

doc/lua_api.md

include/maxwell_common.h

include/maxwell_interface.h

include/param_loader.h

include/silo_output.hpp

share/binde_simplified/params_binde_simplified.lua

src/maxwell_common.cpp

src/maxwell_cpu.cpp

src/maxwell_gpu.cpp

src/maxwell_interface.cpp

src/maxwell_solver.cpp

src/silo_io.cpp