More stringent default tolerance in cosmology.z_at_value.  Doc fixes

* Default tolerance decreased from 1e-5 to 1e-8 in z_at_value;
  redshifts are occasionally measured to better than 1e-5, so it
  makes sense to be more precise by default.  1e-8 seems like
  enough for any practical usage.  This follows the precise by
  default, slow by express instruction principle.
* Change default zmin to a small but non-zero value in z_at_value
  because some of the methods are not defined at z = 0 (like distmod).
* Fix example given in z_at_value docs, which used np.logspace
  incorrectly (it assumed logspace worked the way it -should- work,
  but sadly not the way it actually does).
* Increase stringency of roundtrip z_at_value test, add roundtrip
  test of angular_diameter_distance_z1z2.
* Closes #3008