Here it would be faster to assign the `.value` to `dm1`, `dm2` and `dh_2`:
```
         dm1 = self.comoving_transverse_distance(z1).value
         dm2 = self.comoving_transverse_distance(z2).value
         dh_2 = self._hubble_distance.value ** 2
```
and then convert `out` to a new `Quantity` instance with Mpc units once all the calculations are done.

Mind you, I'm not sure anyone actually uses this method!