[writer] scan group no longer external
After inspection @sole confirmed there is no longer any need to create separate files for scan groups. The writer treats the "scan group" as any other scan: it creates an NXentry that looks like any other scan. This NXentry does not contain softlinks to the scans that are grouped (because that would make it different from any other scan). It does however have an NXdetector called "scan_numbers" so you can figure out which scans are grouped. It would be easy to have an NXcollection under the NXentry which contains softlinks to the scan entrees but as I understand @sole we won't allow this?