Added self.has_missing keyword in initialization of the Dispersion class.
This is to test once, per spin or cluster. This saves a looping over the dipsersion points, when collection the data.
Task #7807 (https://gna.org/task/index.php?7807): Speed-up of dispersion models for Clustered analysis.
Authored by: tlinnet 2014-06-08
Parent: [r23740]
Child: [r23742]
