Detail magnets geometry definition

Current geometry definition for magnets is very simple with two variables MAGU_pos and MAGD_pos to define "effective" magnetic space of two magnets.

The simplified definition is not sufficient for proper tracking.

Proposal: replace MAGU_pos, MAGD_pos, MAG_field with structure like:

struct MagGeometry { TVector3 Start; TVector3 End; double GapWidth; double GapHeight; TVector3 field; }
MagGeometry MAG1;
MagGeometry MAG2;
MagGeometry MAG3;