Use references rather than copying containers in libpbd xml code

It is slightly surprising but there seems to be little difference to
performance with these changes. Possibly a slight improvement in "Create" test
with a large xml document(~5%).

Timing results before these changes with an optimized build using new XML perf tests:

XMLTest::testPerfMediumXMLDocumentTiming
   Create : Count: 10 Min: 38656 Max: 63827 Total: 571228 Avg: 57122 (57 msecs)
   Write : Count: 10 Min: 43594 Max: 49279 Total: 459907 Avg: 45990 (45 msecs)
   Read : Count: 10 Min: 80247 Max: 84912 Total: 827207 Avg: 82720 (82 msecs)

XMLTest::testPerfLargeXMLDocumentTiming
   Create : Count: 10 Min: 230706 Max: 456054 Total: 3850998 Avg: 385099 (385 msecs)
   Write : Count: 10 Min: 312322 Max: 353789 Total: 3264211 Avg: 326421 (326 msecs)
   Read : Count: 10 Min: 573556 Max: 610865 Total: 5951908 Avg: 595190 (595 msecs)

Timing results after these changes:

XMLTest::testPerfMediumXMLDocumentTiming
   Create : Count: 10 Min: 41293 Max: 63746 Total: 564448 Avg: 56444 (56 msecs)
   Write : Count: 10 Min: 42932 Max: 49221 Total: 453955 Avg: 45395 (45 msecs)
   Read : Count: 10 Min: 80160 Max: 84678 Total: 824506 Avg: 82450 (82 msecs)

XMLTest::testPerfLargeXMLDocumentTiming
   Create : Count: 10 Min: 228759 Max: 420236 Total: 3587597 Avg: 358759 (358 msecs)
   Write : Count: 10 Min: 307095 Max: 348767 Total: 3205704 Avg: 320570 (320 msecs)
   Read : Count: 10 Min: 572400 Max: 657219 Total: 5959630 Avg: 595963 (595 msecs)

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