Add a constant function to access UT Sectors

PrStoreUTHit was using findSector for each ut clusters, this function is an usual logn lower bound on a vector<pair<key, sector>>

a second std::vector<sector*> was already there and created with an almost good order to be able to create some offsets during DeUTDetector::initialize and therefore doing an access with an almost constant complexity

maybe it's possible to do something similar with findMat (used by PrStoreFThit) if this one is also a logn getter and if it's a hotspot