Moved the Bloch-McConnell matrix for 2-site exchange into lib/dispersion/ns_matrices.py.
This is for consistency with the other code.
Task #7807 (https://gna.org/task/index.php?7807): Speed-up of dispersion models for Clustered analysis.
Authored by: tlinnet 2014-06-20
Parent: [r24203]
Child: [r24205]
