Calculate and save the weights of a run.

def _calculate_weights(self, this_samples, N):
        """Calculate and save the weights of a run."""

        this_weights = self.weights.append(N)[:,0]

        if self.target_values is None:
            for i in range(N):
                tmp = self.target(this_samples[i]) - self.proposal.evaluate(this_samples[i])
                this_weights[i] = _exp(tmp)
        else:
            this_target_values = self.target_values.append(N)
            for i in range(N):
                this_target_values[i] = self.target(this_samples[i])
                tmp = this_target_values[i] - self.proposal.evaluate(this_samples[i])
                this_weights[i] = _exp(tmp)