relax SVN archive

Expanded the RelaxNoValueError to handle one or two spin IDs as arguments.

This is to better identify which spins or interatomic data containers are deficient.

Changed the dauvergne_protocol.py system test script value.set calls with spin.isotope.

The nuclear isotope is now defined via spin.isotope.

Eliminated a number of the specific API parameters relating to dipole-dipole interactions.

These are now provided by the spin.isotope user function and the dipole_pair user functions. The
eliminated parameters are:
'r' - replaced by dipole_pair.set_dist or dipole_pair.read_dist,
'xh_vect' - replaced by dipole_pair.unit_vectors,
'heteronuc_type' - replaced by spin.isotope,
'proton_type' - replaced by spin.isotope,
'attached_proton' - replaced by dipole_pair.define.

Created the spin.isotope user function.

This is designed to be a permanent replacement for the specific analysis API 'heteronuc_type' and
'proton_type' parameters.

Added the nuclear symbol as a wizard graphic.

Added a set of icons for nuclear or isotope related usage.

Deletion of the structure.vectors user function as it has been superseded by dipole_pair.unit_vectors.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