relax SVN archive

Added the function 'self.get_chemical_name()'.

This returns the chemical name corresponding to the supplied hetID string.

Rewrote the docstring of the 'self.write_pdb_file()' function to include PDB record descriptions.

Desriptions of the PDB records HET, HETNAM, FORMUL, HETATM, TER, CONECT, MASTER, and END including
tables of columns, data type, field, and definition have been added. This info comes from the
"Protein Data Bank Contents Guide: Atomic Coordinate Entry Format Description" version 2.1 (draft),
October 25, 1996.

Fixed the spelling of 'infinite' in the 1.2.8 release details.

A few small edits of the release checklist and fixed the scons target 'user_manual_html'.

Updated the manual and changed the version number to 1.2.8.

Ported r2741 from the 1.2.8 tag (the CHANGES file details for 1.2.8).

The command used was:
svn merge -r2740:2741 svn+ssh://bugman@.../svn/relax/tags/1.2.8

Ported r2741 from the 1.2.8 tag (the CHANGES file details for 1.2.8).

The command used was:
svn merge -r2740:2741 svn+ssh://bugman@.../svn/relax/tags/1.2.8

Updated the CHANGES file for relax version 1.2.8.

relax version 1.2.8.

This is a major bug fix release. All users who encounter problems are recommended to upgrade to
this version. Optimisation tests have also been added to the test suite.

Ported r2738 from the 1.2 line.

The command used was:
svn merge -r2737:2738 svn+ssh://bugman@.../svn/relax/1.2

This fixes the massively important bug #7616 (https://gna.org/bugs/index.php?7616) which is the
failure of the model selection between the global diffusion models when the local tm models are
included.