WIP: Modernize EvtGen

• remove redundant explicit constructor and destructors, prefer implicitly generated ones instead
• improve memory management (fixing a few memory leaks as side effect)
  • use std::unique_ptr instead of raw pointers with explicit delete
  • prefer std::vector over raw heap allocated array
  • prefer objects on the stack in case their lifetime is limited
  • if a class has non-trivial copy constructor, implement a consistent assignment operator