supporting the recommended property for Attributes

[sanbrock]

The recommended property seem to be missing from the xsd implementation of Attributes, although it is done from Fields and Groups.

[prjemian]

• Allowing the recommended xml attribute for nexus attributes definitions#1183

[prjemian]

Is there a NXDL file where this is/would be used? I want to verify the Python code renders this in the documentation.

[benajamin]

Proposal is to allow the "recommended" property (to complement "optional" which can be true or false) to be applied to attributes. This "recommended" property has already been approved for fields and groups, so this will just make attributes more consistent.