relax SVN archive

Minor bugfix for the internal structural object add_model() method.

The internal structural object was being called with self as an argument, which would default to the
chain_id keyword argument. The result would be relax state files with multiple copies of the
internal structural object embedded in the structural XML section.

The internal structure object method _pdb_chain_id_to_mol_index() now uses the CHAIN_ID_LIST string.

This allows for the full PDB chain ID range to be supported.

Converted the chain ID list in the internal structural object to the CHAIN_ID_LIST module variable.

Updated the relax version number to 3.1.5 on the http://www.nmr-relax.com/api/index.html webpage.

Updated the news announcement links for relax version 3.1.5 at http://www.nmr-relax.com/download.html.

The correct link is https://gna.org/forum/forum.php?forum_id=2434.

Updated the relax API documentation at http://www.nmr-relax.com/api/3.1/ to version 3.1.5.

Updated the relax HTML manual at http://www.nmr-relax.com/manual/index.html to version 3.1.5.

Updated the http://www.nmr-relax.com/download.html webpage for relax 3.1.5.

Added the relax 3.1.5 manual to the repository.

Updated the version number to 3.1.5.