Amber force-field parameter data

The Amber project states that the Amber molecular-mechanics force fields are
in the public domain:

  https://ambermd.org/

The bundled GAFF 1.4 numerical parameter table was transcribed verbatim from
Open Babel 2.3.2 data/gaff.dat. The numerical force-field data are covered by
the Amber public-domain statement. No Open Babel source code or atom-typing
rules are incorporated into mmff-rs.

Scientific citation:

  Wang, Wolf, Caldwell, Kollman, and Case, Journal of Computational Chemistry
  2004, 25, 1157-1174. DOI: 10.1002/jcc.20035.

Upstream snapshot:

  https://github.com/openbabel/openbabel/tree/openbabel-2-3-2
  commit bf62b617251080e755c5912dd382aade3272d4dd

This notice documents the provenance and redistribution status of the
parameter data; it does not change the GPL-3.0-or-later license of mmff-rs
source code.
