AppDef for Atom-Probe Microscopy

[sanbrock]

https://github.com/FAIRmat-Experimental/nexus_definitions/tree/fairmat

[prjemian]

The new definition should be added to the contributed_definitions folder of the definitions repository (in a branch or fork). Once ratified by the NIAC (item 3 of NeXus Constitution), it will be moved to the applications folder.

The pull request should be made to the definitions repository.

[sanbrock]

nexusformat/definitions#995