Mag field cache opt

Testing a possible optimization of the zone/mesh field passing to BFieldCache which is done each time a new zone is required by AtlasFieldCache::getField. Must pass the b-field components of z,r,phi for the eight corners of a zone. Testing an explicit initialization of a matrix plot memory copy in BFieldCache - need to pick up corners, and the matrix in BFieldCache is [3][8] whereas locally it is stored as [xx][3].

@christos @amoussa