Get indices of bmus, sorted by their distance from input.

def _get_bmu(self, activations):
        """Get indices of bmus, sorted by their distance from input."""
        # If the neural gas is a recursive neural gas, we need reverse argsort.
        if self.argfunc == 'argmax':
            activations = -activations
        sort = np.argsort(activations, 1)
        return sort.argsort()