Fixed ITrackFitter interface for VectorFitter

It was broken when made thread safe. Basically the ability to fit a set of tracks in one go was lost. It is now back

• [2017-07-15 00:07] Validation started with lhcb-future-clang#250
• [2017-07-15 00:07] Validation started with lhcb-future#466
• [2017-07-16 00:07] Validation started with lhcb-future-clang#251
• [2017-07-16 00:07] Validation started with lhcb-future#467
• [2017-07-17 00:07] Validation started with lhcb-future#468
• [2017-07-17 00:08] Validation started with lhcb-future-clang#252

merged

mentioned in commit abf45372