TrackMatrix API

After #448 (closed) and #184 (closed).

Implementation of SnapshotConst TrackMatrix::snapshot(CaptureBaseConstPtr _capture) const ( and FeatureBaseConstPtrList TrackMatrix::snapshotAsList(CaptureBaseConstPtr _cap) const that calls it) is not efficient.

The class could be improved by indexing snapshots by const pointers (CaptureConstPtr) or by capture's id.