Merger of the dipole_pair and interatomic user function classes.
The functionality of these two classes overlaps significantly. And the dipole_pair user functions
are not related to magnetic dipole-dipole interactions. Therefore all the user functions from both
classes were shifted into the new interatom user function.
This change will affect almost all relax scripts but, as this will form part of the relax 3 release,
script breakage should be expected anyway.