BurstConfiguration-SimulationParameter attribute should support Equations inside

Add methods "from_json", "to_json" for an Equation.
Change BurstConfiguration to keep SimulationParameters as a simple Dictionary (not JSON) all the time, when not stored in DB.
Right before storring in DB convert (calling methods "from_json", "to_json" specific above) SimulationParameters field into a String (add method "prepare_sim_params_before DB" on BurstConfiguration).