pkp/pkp-lib#6251 Add isPreprintOf relation to DOI exports

[ajnyga]

pkp/pkp-lib#6251

[ajnyga]

@ewhanson @NateWr

This is an additional change to the OPS Crossref plugin to introduce isPreprintOf relation (link to the version of record DOI).

There is an older pr for this, but it was delayed because of the ongoing DOI work. See the discussion here where Nate suggest how the OPS DOI plugin should force versioning of DOIs if Crossref is used for registering: https://github.com/pkp/ops/pull/103/files. However, now I've just heard from Alex that this could change. But that is probably something that still needs to be resolved.

In any case this should be merged first, I can rebase this pr after that if needed: #9

[NateWr]

Looks good. Just the one change to code organisation.

[ajnyga]

@NateWr split the function to two parts as requested.

Since #9 is touching the same code, I will wait for this to be merged first.

[NateWr]

Looks good. Are there any other pull requests that need to be merged simultaneously with this one?

[ajnyga]

hmm, why did I not see an alert about this...

There is another one, but it would be easier if this would be merged first and I could rebase the other and pr it then.