`read_nexus` limitations

[RubelMozumder]

NeXus Verification proposed by Florian started in PR #133 which has been superseded by PR #333.

This PR should handle the following issues (which are related to verification)

• Resolving units attribute does not always work #139
• [Bug]: Invalid groups with linked children don't get detected as undocumented #429
• Links create sometimes undocumented attributes #423
• [Bug]: read_nexus fails on illegal AXISNAME_indices #436
• Namefitted attributes are not recognized #422
• Enumerations not in appdef are not checked #434

This is the list of issues that would be resolved in the verification process of the nexus data template.

[lukaspie]

@RubelMozumder all of the remaining open issues here are from read_nexus and not of the validation itself. So I renamed this issue and changed the tags.