added logic to look into the current zone which has been used by the...

This should provide a small cpu improvement for tracking propagation when requesting getField, for 3d field not ZR. The AtlasFieldCache provides a field cache for one "cell" in z,r,phi space. When there is a request to the cache for a point outside the cell, a new cell is fetched from a 'zone/mesh' object which holds the field for many cells in a region of the detector. For example, the solenoid field is in one zone. Up to now, when a point is outside of a cell, the zone for the next cell was requested from the FieldMap, and then the cell for the cache is looked-up inside the zone. We just realized that one can hold onto the zone and first check the zone if the new point is inside the same zone (a simple z,r,phi test) before redoing the look-up for zone from the FieldMap, which often returns the same zone.

There is also some improvements to the documentation in the BFieldMesh class, which was started in !49346 (merged).

This will need a full rebuild to test since AtlasFieldCache.h, .icc have been changed.