update magnetic field

• Add python script to simulate magnetic field following Dave's approach (https://github.com/dwcasper/KalmanFilter/blob/master/Field/BField/__init__.py).
• Fixed charge sign when calculating the initial parameters, since the magnetic field is now pointing in the opposite (negative) x direction.
• Updated conditions to use the new field table ("FaserFieldTable_v2.root")

TODO: the field table still has to be copied to cvmfs. The current version can be found here: /afs/cern.ch/user/t/tboeckh/public/FaserFieldTable_v2.root