Draft new version of METS schema that does not reference XLink

[aelkiss]

Neither PREMIS 3 nor EAD 3 reference XLink. This would resolve several of the XLink related issues: #30, #33, #19.

I will draft a revision of the METS schema that:

• does not reference XLink
• retains the same attributes, but removes the xlink namespace qualification
• still allows existing METS documents that use the xlink namespace to validate
• moves the revision history in comments to documentation elements

[aelkiss]

See https://github.com/mets/METS-schema/compare/noxlink