relax SVN archive

Deletion of the structure.vectors user function as it has been superseded by dipole_pair.unit_vectors.

Only the user function definition has deleted - the backend code will remain so that it can be used
internally.

Implemented the dipole_pair.unit_vectors user function backend.

This code originates from the generic_fns.structure.main.vectors() function (the structure.vectors
user function backend). The dipole_pair.unit_vectors user function is designed to replace
structure.vectors.

Fixed the call to dipole_pair.unit_vectors in the dauvergne_protocol.py system test script.

Refinement of the dipole_pair user function definitions.

Created the dipole_pair.unit_vectors user function definition.

Created the backend of the dipole_pair.read_dist user function.

Created the dipole_pair.read_dist user function definitions.

This new user function is for simplifying the loading of many different interatomic distances into
relax.

Fix for the dipole_pair.define user function - the interatom object can pre-exist, containing other data.

More refinements of the dipole_pair.define user function definition.

Refinement of the dipole_pair user function definitions.