Adding common handling for the magnetic field settings tothe component interface

Adding common handling for the magnetic field settings tothe component interface
Components will use the AliMagF instance in the future. This instance is set up
by the framework
1) AliRoot: AliReconstruction, AliSimulation or explicitely in the macro
2) online environment: AliMagF initialized via the GRPManager from the ExternalInterface

This change will soon make all private code regarding setup of AliMagF obsolete in the
HLT code. Furthermore the SolenoidBz OCDB entry will be removed.