Added option to export in CFF (Citation File Format)

[jeanprbt]

This PR adds the possibility to export BibEntries in CFF format, as requested in #10661. It supports several entries per .cff file, but I don't know if this is permitted by the CFF format. This is hard to solve since the export process in the code is intended for several entries. Also, since the CFF format is primarily intended for pieces of software, this exporter uses credit redirection with the line preferred-citation to specify another type of document.
The fields supported are entry type, author's name, abstract, DOI, volume and URL.

The date is quite complicated since CFF forces date to be of the form "yyyy-mm-dd". This causes problems if only month and year are specified, i.e. 2021-03. It can be solved using a DateFormatter specifically for the CFF exporter and using a default day number, but I did not know if that was necessary, hence I did not implement this.

Documentation is not up-to-date, CFF as export format must be documented.

Closes #10661

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (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 developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Thanks for the contribution!

For date, you could checkout the dateformatter, howeer I don't know how it behaves with missing info. Maybe you can add some more test strings for that (DateFormatterTest)

DateFormatter : formats a date. With no argument, the date is given in ISO-format (yyyy-MM-dd), which is also the expected format of the input. The argument may contain yyyy, MM, and dd in any combination. For example \format[DateFormatter(MM/yyyy)]{\date} will output 07/2016 if the date field contains 2016-07-15.

https://docs.jabref.org/collaborative-work/export/customexports

[jeanprbt]

I fixed the date and implemented this field when exporting to CFF.
Now, when a date contains year, month and day, it is displayed as following :

date-released: yyyy-mm-dd

It it only contains month and year, it is displayed as follows :

 month: mm
 year: yyyy

It is only contains year, it is displayed as follows:

year: yyyy

Finally, if it is poorly formatted, is is displayed as follows:

issue-date: <date-as-is>

This allows not to invent a day and adapt the format to acceptable CFF fields if day or month are missing. Hope you're happy with that !

[Siedlerchr]

Looks good to me now, I'm okay with the pragmatic date approach

[Siedlerchr]

A bit out of scope, but if you ever need to parse more complicated date formats you can see Jabref's Date class which contains some more crazy formats

[koppor]

Thank you for the contribution.

Some small comments.

Do you have time to craft a roundtrip test? We now have an exporter and an importer.

[jeanprbt]

I do not have much time to implement a roundtrip test, but I think this is more complicated than it seems. In fact, the importer only handles two types of entries, software and dataset. Because the exporter supports a much wider range of entries, using the preferred-citation reference block, this is hard to get the same input and output files.
Furthermore, I did not find any way to extract the first name of an author separately from his last name. This means I am forced to use the field name for authors, whereas many .cff files use separate last-name and given-names fields.
But the exporter is conform to the .cff format right now, and I fixed all your requested changes. :)

[Siedlerchr]

I'm okay with that for the moment. Then we create a new task for roundtrip test.
For author parsing:

jabref/src/main/java/org/jabref/model/entry/AuthorList.java

Lines 165 to 172 in 79a7756

	* Retrieve an AuthorList for the given string of authors or editors.
	* <p>
	* This function tries to cache the parsed AuthorLists by the string passed in.
	*
	* @param authors The string of authors or editors in bibtex format to parse.
	* @return An AuthorList object representing the given authors.
	*/
	public static AuthorList parse(final String authors) {

[Raahitya-14]

Thank you for the contribution.

Some small comments.

Do you have time to craft a roundtrip test? We now have an exporter and an importer.

Hey, I am Jean's friend, I can work on crafting a round-trip test! Please let me know if that's fine.

[koppor]

@Raahitya-14 Sure, go ahead! Maybe the importer has to be improved, too.

As first test https://github.com/JabRef/jabref/blob/main/CITATION.cff Should be imported. Maybe, your work turns out to improve the importer...

[Siedlerchr]

@Raahitya-14 I suggest you create a new PR for this then

[Raahitya-14]

@Raahitya-14 I suggest you create a new PR for this then

Sure, I will create a new PR