relax SVN archive

Added initial support for the residue user function class.

The file 'prompt/residue.py' was copied from 'prompt/sequence.py' and the function 'add()' renamed
to 'create' (generating the 'residue.create()' user function). All the other sequence user
functions still remain unchanged.

Completion of point 9 of the preparation in the data model redesign document.

Simplified the chi2 gradient unit test and added delete statements to destroy all unit test data.

Added a tearDown statement to delete the chi-squared Hessian.

More docstring improvements for the chi-square function unit tests.

Added a unit test for the chi-squared Hessian function d2chi2.

Improvements to the docstrings and a comment in the chi-squared value and gradient unit tests.

Replaced the empty chi2 and dchi2 unit tests with true unit tests for those functions.

Removed the line '#!/usr/bin/env python' from all the unit test modules.

Removed the svn:executable property from the 'maths_fns/test_chi2.py' unit test file.