This is a rewrite of Frank Timmes' aprox21 network, based on aprox21.tbz
as downloaded from his website on 2015-12-31. It follows the same reasoning
as the aprox13 network in this repository, so see the README in that directory
for information on our integration strategy. This directory is a copy of the
files in the aprox19/ directory, with appropriate changes for the number of
species and reaction rates.

We thank Frank for allowing us to redistribute these routines.
