turn FTSFromVertexToPointFactory::operator() into a static method

FTSFromVertexToPointFactory does not have any data members, and a single (currently non-const) method, FTSFromVertexToPointFactory::operator() .
This patch changes that single method into a static method, FTSFromVertexToPointFactory::get(), and propagates the change to downstream code.
Merge pull request #1203 from fwyzard/make_FTSFromVertexToPointFactory_static

Multithreading fixes -- turn FTSFromVertexToPointFactory::operator() into a static method