code: def beta_r(self, r, kwargs):
          
          if self._type == 'const':
              return self.const_beta(kwargs)
          elif self._type == 'r_ani':
              return self.beta_r_ani(r, kwargs)
          else:
              raise ValueError('anisotropy type %s not supported!' % self._type)
docs: returns the anisotorpy parameter at a given radius
          :param r:
          :return: