Add support for .nxs files

[adas42]

It would be really useful to add .nxs (nexus) files to the list of file formats. Nexus files can utilize the .h5 reader and are a common output file format for CHAP (CHESS Analysis Pipeline). My current workaround is to rename the .nxs to .h5 and then import into tomviz but this breaks other workflows.

[psavery]

Sounds good! We will hopefully have funding very soon to fix this!