ESD can init the Bfield via AliESDRun::InitMagneticField()

ESD can init the Bfield via AliESDRun::InitMagneticField()
1) Added currents/beam info to AliESDRun and the method
to AliESDRun::InitMagneticField() (+ alias AliESDEvent::InitMagneticField())
2) AliReconstruction will fill needed Bfield info in AliESDRun
3) AliMagF got a "static CreateFieldMap" to create a map from the
currents. All classes which create the field (AliGRPManager,
AliReconstruction, AliESDRun) will use this method