Update medline xsd schema to include mathml and other stuff

[Siedlerchr]

Fixes #4273

I managed to generate the xsd form the main dtd and the additonal dtds

Currently jaxb/xjc fails due to some duplicate property in the mathml files.

I don't quite understand how to fix this, it must be somehow annoated with some other jaxb tags

Something like this, but I have no idea where exactly to place it. If I try to place it directly under the reported lines it is invalid.

        <xs:annotation>
                <xs:appinfo>
                    <jxb:property name="multiscriptExpr"/>
                </xs:appinfo>
            </xs:annotation>

https://javaee.github.io/jaxb-v2/doc/user-guide/ch03.html

Some related info:
https://cumbers.wordpress.com/2009/09/02/property-value-is-already-defined-use-to-resolve-this-conflict/
highsource/jsonix-schema-compiler#83

[ant:xjc] [ERROR] Eigenschaft "MultiScriptExpression" ist bereits definiert. Verwenden Sie <jaxb:property> zur L▒sung dieses Konflikts.
[ant:xjc]   Zeile 1687 von /jabref/src/main/resources/xjc/medline/mathml3-presentation.xsd
[ant:xjc]
[ant:xjc] [ERROR] Das folgende Verzeichnis ist f▒r den obigen Fehler relevant
[ant:xjc]   Zeile 1690 von /jabref/src/main/resources/xjc/medline/mathml3-presentation.xsd
[ant:xjc]

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[koppor]

Could you please provide more details what you generated how? I think, pointing to the PubMed XML examples is helpful to follow the intention here.

Generating XML Schema from example seems to be a work-around for non-existing XML Schema files. This is like generationg JSON Schema for JSON files not under own control. Or specifyng required packages meassures for received packages (and no power on the producer).

In case this causes much effort, I would propose to rewrite the Medline importer using the StAX technology (https://www.baeldung.com/java-stax). We could mob on that.

Minor note: In another project, I found following plugin great: https://github.com/edeandrea/xjc-generation-gradle-plugin

[koppor]

Why is the medline.xsd was regenerated? These codes do not exist in the new file. -- Think, there has to be another way to achieve the goal.

[Siedlerchr]

I commited an old version. I managed to recreate the xsd now from the dtd and a bunch of other required dtds and adjusting the headers. However, the orginal error regarding the mathml remains.

We already use our own xjc plugin which does also execute the ant task.