Returns the number of samples in the input data
            @ In, key, an optional 2-tuple specifying a min-max id pair
            used for determining which partition size should be
            returned. If not specified then the size of the entire data
            set will be returned.
            @ Out, an integer specifying the number of samples.

def get_sample_size(self, key=None):
        """ Returns the number of samples in the input data
            @ In, key, an optional 2-tuple specifying a min-max id pair
            used for determining which partition size should be
            returned. If not specified then the size of the entire data
            set will be returned.
            @ Out, an integer specifying the number of samples.
        """
        if key is None:
            return len(self.Y)
        else:
            return len(self.get_partitions(self.persistence)[key])