Separate (re)setting ChordFinder from FieldBuilder, percolate change to all physics lists

Something like this is needed to use magnetic field in the worker
threads.
Setup magnetic field in RunManagerMTWorker
Move function-statics to thread_local in Field
Add customiseMultithreadedSim to easily switch to OscarMTProducer (and multiple streams) from cmsDriver.py/runTheMatrix.py
Fix statics in G4MonopoleTransportation

One to thread_local, remove the other as unnecessary.
Remove unused static variable from TrackWithHistory