Fixed bug where we could return a const & to a temporary from ParameterSet::getUntrackedParameterSet*

The recent change to getUntrackedParameterSet* to no longer register the default value
to the Registry lead to the bug where if the user passed in a temporary as the default then
that temporary could be returned by const&. If the user's receiving variable is also a const&
then one has an address to a temporary which the compiler can delete after that C++ line ends.