Approve clarification of gain_setting in NXdetector

[benajamin]

The proposal is to approve pull request https://github.com/nexusformat/definitions/pull/896 to allow free choice of values in the gain_setting field of NXdetector.

Current NXDL:

 <doc>The gain setting of the detector. This influences background etc.</doc>
    <enumeration>
      <item value="high"/>
      <item value="standard"/>
      <item value="fast"/>
      <item value="auto"/>
    </enumeration>
 </doc>

Proposed NXDL:

  <doc>
      The gain setting of the detector. This is a detector-specific value
      meant to document the gain setting of the detector during data
      collection, for detectors with multiple available gain settings.
  </doc>

[prjemian]

We should continue to list (now in the documentation if this is approved) the expected values.

[benajamin]

Consensus is that this would be much improved by including a list of recommended strings. We would like to consider a revision in the near future.

[phyy-nx]

Ready for further review after addressing feedback from Spring NIAC 2022

[benajamin]

Proposal is to include the following list of recommended string values:

• "standard"
• "fast"
• "auto"
• "high"
• "medium"
• "low"
• "mixed high-medium"
• "mixed medium-low"

[benajamin]

The approved changes have been implemented in https://github.com/nexusformat/definitions/pull/896