PointFeature: add copy operator

Change-Id: I8812f331c2152f4327384fe5805c4dbc40632a0d
KeypointFeature: add proper setter and virtual constructor

Change-Id: I2ec75edade21f746621e7c1b1bb2983e066a0f43
Tracker::Track use proper setter for setting KeypointFeature

Change-Id: I5669bc39987390ebf204fab90a8eeb3eec71f689
Tracker::Track: fix accessing invalid element

it can not be assumed that both images have the same number of features

Change-Id: I1716203619d9fa7b6ca42453975e44ebe626ce6f