[Bug]: Validation of open enumerations does not search for `@custom` attribute

[lukaspie]

Contact Details

No response

What happened?

When open enumerations were introduced in nexusformat/definitions#1521, it was decided that

• for fields, an attribute @custom=True.
• for attributes, an additional attribute @my_attribute_custom=True (where my_attribute is the name of the attribute with the open enumeration)

shall be added to the file. In that case, only an info shall be logged, whereas if these attributes are missing, a warning is expected. We never actually implemented that in the validation.