Better interface for magnetic field access

In #97 , the magnetic field (at least one double for Bz) is stored in the metadata. As noted in https://github.com/key4hep/k4SimDelphes/pull/97#issuecomment-1339276842 , this information would ideally be accessed via the geometry service. The metadata is added only as a workaround until a proper solution is found.

Start by reading issue #97 and the linked pull-request discussion, especially how the magnetic field is currently stored in metadata. Then identify the geometry service interface and determine how it should expose the magnetic field instead. Done means the metadata workaround is replaced by a defined geometry-service access path.

Written by the indexing model from the issue text.