Added DeMagnet Detector element with Basic access to magnetic field

Ben Couturierrequested to merge
bcouturi_DeMagnet into master

Added DeMagnet Detector element with Basic access to magnetic field, from the DeMagnet itself.

The MagneticFieldGrid and MagneticFieldGridReader have been migrated from LHCb, thus requirinbg Vc to be added as a dependency of Detector.

Edited by Ben Couturier

Merge request reports