bugfix in scaling of magnetic field with currents

The update replaces the hardcoded scaling factors with those calculated from configured and nominal currents, using the following configuration (example meant to be used for studies related to special run with low B field, ATLIDTRKCP-380) :

from AthenaCommon import CfgMgr
CfgMgr.StandardFieldSvc.MapSoleCurrent = 2000 

To avoid failures at the moment when maps with lower-than-nominal current are created, the nominal map currents are retrieved from the name of field map input file before the scaling is done. It would be safer to read the map current from the root file rather than from its name, please advice.

The issue is tracked by Jira ticket ATLASSIM-5485