MCParticle linked to MCCaloHits created from beam hole library + improvement of StoreCaloHitsAlg class

In the previous version of GetFastCaloHitsAlg, the MCCaloHits created from hits in the beam hole library were associated with a null MCParticle pointer. This caused a crash in LHCb::Calo::Algorithm::CaloDigitLinker::operator(). The correct solution is not to allow null MCParticle pointers to be associated with MCCaloHits. Instead, the new MCCaloHits should be associated with an existing MCParticle that has typical characteristics of particles normally linked to Geant4-based MCCaloHits. This solution has now been implemented.

Changes also include improvements to Sim/GaussFastSim/src/StoreCaloHitsAlg.cpp, removing the limitation on the maximum number of stored hits. In addition, the handling of the output ROOT file has been improved.