Manager for a field as part of the RNTupleProcessor.
An RFieldContext contains two fields: a proto-field which is not connected to any page source but serves as the blueprint for this particular field, and a concrete field that is connected to the page source currently connected to the RNTupleProcessor for reading. When a new page source is connected, the current concrete field gets reset. A new concrete field that is connected to this new page source is subsequently created from the proto-field.