add compatibility with GaudiUtils::VectorMap changes

Make RecSummary forward (and backward) compatible with changes to VectorMap.

This by itself does nothing (*) observable, but will be required for gaudi/Gaudi!1838 to read back older versions of RecSummary.

(*) to be pedantic: it will read a VectorMap from disk into a separate instance, and then make a copy to the data member - so there will be an extra copy operation.