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 assertScalar(obj)
    if ~isscalar(obj)
        error('sbplib:assertScalar:notScalar', '"%s" must be scalar, found size "%s"', inputname(1), toString(size(obj)));
    end
end