Use references rather than copying containers in libpbd xml code

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)