remove 'oracle' option from indirectOpen

The 'oracle' parameter of AtlCoolLib.indirectOpen turns out to be a bit misleading: Its original intention was 'no DBRelease' while most people nowadays rather understand 'no frontier'. Moreover, the underlying if-else logic looks wrong to me.

IMHO that best way forward is to remove the option entirly. This MR does this and adjust (hopefully) all clients.

Moreover: Introduce a cache in MagFieldUtils to avoid multiple DB lookups.