Can now load large mcstas nexus files. re #9137

Updated test do check that can the same results before and after.

Reading in blocks of about 50MB. This seems to work quite well on my windows system, but perhaps another value will could give a better performance

Note now that reading in part progress reporting could be improved. Not done here
Remove compiler warning. re #9137
Merge branch 'feature/9137_large_files' into develop