message: adding setExpectationValueZ method to buffer
diff: @@ -143,6 +143,11 @@ public:
  return aver;
  }
  + virtual void setExpectationValueZ(const double exp) {
  + XACCLogger::instance()->error("AcceleratorBuffer.setExpectationValueZ not "
  + "implemented. This method is intended for subclasses.");
  + }
  +
  /**
  * Return a read-only view of this Buffer's measurement results
  *
  