TrackFindingAlg const correctness for CKF_pimpl

Apart from the CKF_pimpl constructor (inheriting CKF_config), TrackFindingAlg now only ever accesses the object via trackFinder() accessor functions. This ensures that execute() (and the const member functions that it calls) can't mistakenly modify CKF_config.

This change was extracted from the largely unrelated !64989 (closed).