add user-agent field for indicating generator / serialization tool used

[cboettig]

Optional field to indicate if the EML file was generated by Morpho, the R EML package, or some other tool; including the tool version. (e.g. static website generators often seem to use the meta tag "generator"). I believe there's currently no obvious mechanism for this.

[mbjones]

Great idea. I retitled this and added it to the EML 2.2 milestone and project.

[mbjones]

ISO 19115 also has some metadata-related properties that EML is missing, such as metadata modified date. Maybe those would belong in the same section? To be discussed. And I see that there is already issue #146 for these metadata maintenance fields.