Geometry and configuration of Magnetic Field from DB

Infrastructure to take the MF geometry and configuration from DB instead than from python cfg. 
Changes include:
- Specific record for MF geometry (MFGeometryFileRcd), do not use GeometryFileRcd anymore. 
- New DB payload for MF configuration (MagFieldConfig); incl. record, tools to create it, etc.
- Prototype of new ESProducer to use the above (no change in behaviour for existing producers)
- Some refactoring in MagneticField to allow construction of parametrized fields at runtime
- removal of one large reference file as reuested by Slava (cf #5269)

This PR introduces no change in standard configurations (regression tested). Plan to switch to the new mechanism in a future PR, once the DB is populated with the new payloads.

Relevant Discussion:
https://indico.cern.ch/event/328166/session/1/contribution/24
@namapane 
I realise this just now: why have you removed `DTRecoUncertainties` from the list of known Condition classes?
Just a leftover?