Adding a keyword to the input file

Follow the instructions to add a new keyword:

• Add the new item in "include/SimulationParams.h"
• Initialise and destroy the new item in SimulationParams::SimulationParams() and SimulationParams::~SimulationParams()
• Read in the value in SimulationParams::assign
• Validate the value in SimulationParams::validate
• Go to World::init and manage your new item to be used to initialise your new stuff.