Annihilation condition checks the cross products of the Burgers vectors
and line vectors
In order to circumvent the effect of local co-ordinate systems of the
dislocations, the annihilation condition is set to be one of opposing
cross products of the Burgers vectors and Line vectors.
The dislocations created by DislocationSource::emitDipole are given
their Burgers and line vectors explicitly.
Explicit definitions of the Burgers and line vectors are provided.