NeXusIOHelper using non-type templates and enum for Narrowing

[rboston628]

Description of work

Summary of work

When implemented, NeXusIOHelper had used two empty structs to manage narrowing or not, passed as a template type parameter.

Since these structs were just acting as a boolean flag, it makes more sense to reimplement these as a boolean flag through an enum with non-type template parameters.

This also deletes the callGetSlab etc. methods, which were only used to wrap closing the file after the call. Closing is better handled by the calling function, and can be put in a separate block at the end of the caller.

Part of ongoing #38332

Further detail of work

To test:

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[peterfpeterson]

Changing to an enum for narrowing and cleaning up the if/else trees is appreciated.