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 benchmark(func, N)
    default_arg('N',100);

    tic
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    for i = 1:N
        func();
    end

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end