Make countrate_correction_lookup_table optional in NXmx?

[phyy-nx]

As pointed out to me by @biochem-fan, countrate_correction_lookup_table is not applicable to CCDs so it doesn't make sense to make it a required attribute. I suppose this similarly applies to countrate_correction_applied. These NXdetector fields were added as required to NXmx in #975.

[soph-dec]

I think the fact that the countrate_correction_lookup_table is required was a simple oversight by me (in NXdetector, not specifying makes a field optional, while in NXmx, it defaults to required). It does not make much sense to have it required since countrate_correction_applied is optional.