CodeMeta export issues

[dgarijo]

Jael Castro reported a few issues:

• Codemeta namespace is not used (double check)
• build instructions is not correctly identified.

[dgarijo]

Additional issues:

• License returns a name and a url (correct), but we don't return the spdx identifier.
• issueTracker has a /number which is problematic
• releaseNotes should not be empty
• author should be maintainer
• some properties are not in codemeta: licenseText, fullName, stargazers, scriptFile.

[dgarijo]

Also multiple descriptions are allowed, in theory, but we may add a flag to conflate.