Topic/multiple sift per object

Hi,

This PR allows to define multiple sift files associated with an object. This can be used to handle important scale changes but could also be used to detect multiple objects that share the same ply file but have different textures (e.g. a set of tea boxes). Only the former is currently considered, so if multiple sift are associated to an object and the detector reports multiple successful finds then only the one with the highest confidence is selected.

Finally, the way the objects are read in the tracking.ini file is a bit different, whereas before the Model/SiftModel were associated following their index in the string, it is now based on the model name (that is the ply model name without the ply extension) so that if the object is name "object" then it will be associated to all sift (in tracking.ini) that match the "*object*" pattern.

Best,