Spinning off from #1407 (comment), the idea is to allow for open enumerations, i.e., enumeration to allow values not explicitly given in the list. An example is the type field in NXsource (https://manual.nexusformat.org/classes/base_classes/NXsource.html#nxsource-type-field) which is too restrictive for some experiments.

Note that open enumeration should not be the default as for some fields/attributes, the values given in the enumerations really are the only sensible ones (a prominent example being NXentry/definition field in most application definitions.

Tasks:

• Create a new PR that adds to enumeration an attribute open=False, which would allow the enumeration to allow values not in the list.
• Adapt sphinx to render it nicely (it should be possible to see from the html if the enumeration is open or closed). Could potentially be done in the same PR.
• Check implications for validators. They should not produce errors but could produce a warning.