Computation of nx_data_mean for NeXus/HDF5 uint8 arrays raises

[mkuehbach]

NeXus/HDF5 files from NXapm_paraprobe_results_* and NXmicrostructure_score_results use uint8 arrays for storing flag and bitpacked boolean state information. Upon drag-and-drop parsing these into NOMAD, (search quantity sprint branches) the NeXus parser tries to compute mean values from these uint8 integers. Technically this should work but this raises an error causing the parsing pipeline to stop where a warning would be appropriate. Uint8 should be supported as it can be both promoted to >8bit uints or >8bit ints. Note that means of these uints are meaningless, because of which indeed there are cases where NOMAD should not greedily try to index everything. Of course one could write two files but that is a naive workaround.