Improve THcShower class structure and cleanup

  Clean up THcShower.h and THcShower.cxx.
  typedef vector<THcShowerHit*> THcShowerHitList by
  typedef set<THcShowerHit*> THcShowerHitList.
  Remove inheritance of class THcShowerCluster from THcShowerHitList.
  Define THcShowerCluster as THcShowerHitList, aka set<THcShowerHit*>.
  Remove inheritance of class THcShowerClusterList from
        THcShClusterList, a.k.a. vector<THcShowerCluster*> container.
  Define THcShowerClusterList as vector<THcShowerCluster*>.
  Rename THcShowerHitSet by THcShowerHitList.
  Rename HitList (related to the HMS calorimeter) to HitSet.
  Add operator< in THcShowerHit class, in order to have THcShowerHitSet
    objects to be properly sorted.