refs #28 but not fully NeXus compliant

makes existing structure almost NeXus compliant

still needs a hard link from /entry/instrument/detector/data to /entry/data/data

need support software interface to create NDF5 hard links from specifications in an XML file
I would prefer not to add NeXus tags to this group in the 'default' XML. The reason being that it only contains file writer performance measurements - measuring the performance of writer and file system. This is not science related data, but rather engineering (debugging) information and not of interest to the scientists.