add license URL to EML

[mbjones]

EML currently supports a textual description of usage rights, but lacks a clear, machine-readable declaration of the license for a data set. Consider adding an optional field for licenseURL or an SPDX license key that makes it simple for end users to be explicit about which license they follow. See discussion in codemeta/codemeta#67 .

[mbjones]

Added a new licensed element and LicenseType. See SHA 491b715. And here's an example of it in use:

<licensed>
    <licenseName>Apache License 2.0</licenseName>
    <url>https://spdx.org/licenses/Apache-2.0.html</url>
    <identifier>Apache-2.0</identifier>
</licensed>

Note that the element is named licensed because the element name license was already in use in eml-software for the text of a software license. I decided to leave the software elements in place for backwards compatibility.

[mbjones]

@cjones @jgoldstein @gothub @cboettig Please review for compatibility with the licensing needs for Arctic Data Center and with the CodeMeta licensing approach. Thanks.

[csjx]

👍 This looks good to me. I reviewed the Creative Commons embedding guide to see if we are in alignment. Using RDFa it seems like they express the same information that we do between both the usageRights element and this licensed element. They emphasize i18n of license descriptions, but that seems more to do with usageRights for EML. They also explicitly point to a license image, but I'm not too concerned about that. Ship it ™️ .