Added support for reader/writing composite field data.

The XML readers and writers did not previously support reading and writing of field data in composite data. Added support. This is a first pass at this. Only field data of the root node is supported and it is always written inline - that composite files do not support appended data. This should be used for meta-data mainly not large field data.

Also changed the composite pipeline to copy field data for composite data when the filter is not composite aware. Previously the output was initialized and field data was lost.