relax SVN archive

Created the RelaxInteratomError and RelaxNoInteratomError classes for interatomic data errors.

The model-free analysis is now fully using the new interatomic data container design.

The interatomic data container now has the dipole_pair flag initialised to False.

Started to convert the model-free analysis code to the new interatomic data container design.

Expanded the return_interatom() function to handle a single spin ID.

This function now returns a list of matching interatomic data containers.

Modified the check_args() method of the dauvergne_protocol model-free auto-analysis for the new data.

The heteronuc_type and proton_type are now in the spin's isotope variable, and the bond length 'r'
is now the interatomic distance variable 'r'. All spin containers and interatomic data containers
are being checked.

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.