Add general Runge-Kutta class - Add a general Runge-Kutta class which time integrates the solution based on coefficients obtained from a Butcher tableau - Add butcher tableau which returns coefficents for the specified Runge-Kutta method - Remove RungKutta4proper, since obsolete

function default_struct(s, val)
    if evalin('caller',sprintf('~exist(''%s'',''var'')',s))
        given = [];
    else
        given = evalin('caller', s);
    end

    final = copyWithDefault(given, val);
    assignin('caller', s, final);
end