Validate exported nexus file.

[YooSunYoung]

essnmx should save reduced data as a nexus file for other analysis software.
This feature is done by #21 but it does not check if the nexus file is valid or not.
It should have a test or an option to check if the result file is valid or not.
Maybe chexus can be used here.

It has relatively simple structure, so it shouldn't be so difficult to implement.

[SimonHeybrock]

Note that Chexus is not really checking for validity but for interpretability. It is also mainly targeted at raw data. So I am not sure it would be of much use. Maybe trying to load the file with relevant software would be a more suitable check?

[YooSunYoung]

Yeah, Justin was doing it manually.

The next step is done by dials and it's not distributed on common channels,
like pip or conda as far as I understood.
And the software is >300 MB to download, so I thought it's not suitable for CI test...