Fix memory leaks in GeoModeXml

Valgrind reports many leaks in GeoModelXml due to explicit strings being passed to XMLString:::transcode within function calls, which creates an XMLCh which needs to be released but currently are not. This MR updates all such instances to instead define a temporary XMLCh * variable which gets passed instead, and is then released once it is no longer needed.

In future it may be better to provide a wrapper function, which cleans up behind-the-scenes, to replace all explicit calls to XMLString:::transcode in GeoModelXml.

Before:

==5157==    definitely lost: 54,460,276 bytes in 3,964,688 blocks

After:

==42833==    definitely lost: 1,185,328 bytes in 105,116 blocks

FYI @hessey @dellacqu