Instantiating GeneralGammaDistributionPositive

[alekssmolkovic]

Currently instantiating the GeneralGammaDistributionPositive class from flavio.statistics.probability results in an AttributeError due to the use of self.counts_total here: https://github.com/flav-io/flavio/blob/14982f8cd0e989b36ff10be6d30079b4a659f101/flavio/statistics/probability.py#L1027C53-L1027C70.

[DavidMStraub]

@peterstangl it looks like a unit test for this class was missed in #210, right?