Additional validation

[cboettig]

EML validation imposes additional constraints beyond the XML schema validation. I believe these can be summarized as follows:

• combination of id+system attributes on any element should be unique.
• an element cannot have both child element <references>/<describes> and an attribute called id.
• semantic annotation needs a subject: either from the id of its parent or as a child <references> of the annotation, but not both!

eml_validate() should enforce both of these conditions as well. (Should be possible as simple xpath?)

Migrated from ropensci/EML#244.

[mbjones]

The validation rules in EML are now clearly listed here: https://github.com/NCEAS/eml/blob/BRANCH_EML_2_2/docs/eml-validation-refs.md

That document will be part of the EML 2.2.0 release specification.

[cboettig]

closed by #26