[WIP] Accumulator for cloned Algs

see GAUDI-1194

this provides a mechanism for cloned Algorithms to perform some accumulation/merging across all instances. The user must have their Algorithm inherit from IAccumulator, which defines an operator += function. Just before finalization, the AlgorithmManager loops over all Algorithms that inherit from IAccumulator, and calls += on them, adding Algorithm::index >0 to Algorithm::index=0, ie the accumulated values will show up in the instance of the Algorithm where Algorithm::index() == 0

During Algorithm::finalize(), the user should test for (index()==0), and perform any required special actions.

an example of user code:

class MyAlg : public Algorithm, public IAccumulator {
  public:
   ...
   int val() const { return m_val; }
   IAccumulator& operator +=(const IAccumulator& other);
   ...

  private: 
   int m_val;  // value to be accumulated
}


IAccumulator& MyAlg::operator+=(const IAccumulator& other) {
   auto oth = dynamic_cast<const MyAlg*>(&other);
   if (oth != 0) { /// in reality this should never fail
       m_val += oth->val();
   }
}

StatusCode MyAlg::finalize() {
   if (index() == 0) {
      // accumulated value is in primary instance, ie Algorithm::index() == 0
      info() << "accumulated value is: " << val() << endmsg;
   }
}