looser cuts in Acts Track Finding branch stopper

The track selection minimum p_T and maximum |η| cuts performed in the branch stopper are applied to tracks with poorer resolution than the final track. This MR uses looser cuts in the branch stopper, by default 0.9 times the p_T cut (eg. p_T≥0.81 GeV in the barrel) and and extra 0.1 in |η| (ie. |η|<4.1).

The previous behaviour can be restored for comparison purposes by setting flags.Acts.doTrackFindingTrackSelector=3.

This should fix one source of inefficiency spotted by @astefl.