beta parameter in qgaussian

[giadarol]

The implementation of this method seems not correct (valid only for q=1)

    @staticmethod
    def sqrt_beta(sigma, mathlib=MathlibDefault):
        assert sigma > 0
        return 1 / (mathlib.sqrt(2) * sigma)

The general expression should be implemented (see wikipedia):

The cases q>5/3 for which the standard deviation of distribution is not defined are not of interest for spacechargs studies.