unstructured_three_d_fluid.cc: Added more time stepping parameters

Now able to set:
CommandLineArgs::specify_command_line_flag("--t_min", &RNS::T_min);
CommandLineArgs::specify_command_line_flag("--t_max", &RNS::T_max);
CommandLineArgs::specify_command_line_flag("--dt", &RNS::Dt);
CommandLineArgs::specify_command_line_flag("--ntsteps", &RNS::Ntsteps);