Allow chardata in <codemeta:affiliation>, to be compatible with existing metadata

Manually validate <codemeta:affiliation>. Unfortunately, this cannot be
validated by codemeta.xsd, because Codemeta has conflicting requirements:

1. https://codemeta.github.io/terms/ requires it to be Text (represented by simple content), but
2. https://doi.org/10.5063/SCHEMA/CODEMETA-2.0 requires it to be an Organization (represented by complex content)

See https://github.com/codemeta/codemeta/pull/239 for a discussion about
this issue.