Clarify requirements for @id in attribute elements when child annotations are present

[gremau]

In the <attributes> section of the EML BP, the text states that @id elements are required only if the attribute will be referenced by an annotation or other EML element. This doesn't clearly represent the fact that there is an EML 2.2.0 requirement that any element containing a child <annotation> must have a unique @id attribute, with some exceptions when an annotation is in an <annotations> list references other elements. See the EML documentation here and here for more information. Since this requirement is not defined in the EML schema (XSD) it is difficult to catch this error with standard schema validators. There are a few ways to check this (EML R package, emlvp, and NCEAS EML parser). This WILL cause an error in the EDI congruence checker since it uses the NCEAS EML parser.

**Description and title edited in response to discussion below

[twhiteaker]

Where is that declared in the EML documentation? It seems unnecessary to require it for all attributes if only some have annotations.

[gremau]

You're right @twhiteaker ! I did some testing and the requirement is that any element that contains a child <annotation>annotation must have an @id attribute, not every attribute in the parent <attributeList>. So you can have an <attributeList> containing attribute elements with and without @id attributes. Editing the issue description & title, but still think this needs clarification in the EML BP doc.

The line describing this in the EML schema docs is in Chapter 6: "Elements which contain an annotation child element MUST contain an id attribute, unless the containing annotation element contains a references attribute"