Make a static data member non-static and declare another static const.

The base class for the mixing modules has a static non-const data member, "vertexoffset".  There is no reason that this data member needs to be static.  Even in the unlikely case of multiple instances of the mixing module in a job, the value can be set separately for each.  So this pull request changes the static data member "vertexoffset" to a non-static data member vertexOffset_.

This pull request also declares the static DigiAccumulatorMixModFactory::singleInstance_  to be const.