Moved the Bloch-McConnell matrix for 3-site exchange, into the lib/dispersion/ns_matrices.py.
This is to standard the 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: [r24206]
Child: [r24208]
