[ESSNMX] Fix NXlauetof nexus definition

[YooSunYoung]

There were couple of typo/faults:

• definition -> ~definitions.
• 'entry' should be NXentry, not NXlauetof

I fixed them and

• Refactored nexus writing helper to use the NXlauetof data class instead
• Updated the frozen file

Another minor fix in the PR:

• Updated test helper