Re #6518. Messing with NexusFileIO moving to nexus cpp api.
Re #6518. Moving NexusFileIO to use NeXus cpp api.
Re #6518. Removed public file handle from NexusFileIO.
Re #6518. Moving SaveToSNSHistogramNexus to use NeXus cpp api.
Re #6518. Moving SaveISISNexus to use NeXus cpp api.
Re #6518. Moving NexusClasses to use NeXus cpp api.