Fixing LR Search

- first steps of fixing LR Search (not yet functional)
- generalizing LR to work with any intial subset as seed (previously
sampled first subset of size 2 using exhaustive or random search)
- moved stop criteria to Abstract Neighbourhood Search (do not apply to
other engines like LR, exhaustive and random)
- generalized IntegerSubsetGenerator to work with any IndexType
- updated tests