Made static a member variable of RecoTauVertexAssociator

A static was being used as if it were a member data. This was being
flagged by the static analyzer. It was trivial to make the variable
an actual member variable.
Merge pull request #3621 from Dr15Jones/makeStaticAMemberOfRecoTauVertexAssociator

Multithreading -- Made static a member variable of RecoTauVertexAssociator