* cosmo.el (cosmo--trapzd, cosmo-qsimp): Check Simpson's rule for
  convergence and stop if the desired accuracy is reached. The
  previous algorithm did't perform any convergence check. At low
  redshift z~1 numerical integrals are now about one order of
  magnitude faster, at large redshift z~1 one order of magnitude
  slower (but it is guaranteed to reach the desired accuracy).

* cosmo.el (cosmo, cosmo-int-prec, cosmo-int-maxsteps): Define
  customization group to allow users to set numerical integrals
  precision variable through the `customize' command.

* README.org: Explain how to customize variables.