Introduce a new links category for the YAML definitions

[tmadlener]

BEGINRELEASENOTES

• Introduce a new links category into the YAML grammar to automate the declaration of Links.

ENDRELEASENOTES

• Includes Add templated links between arbitrary datatypes #257
• Documentation
• Includes Make sure that link weights are stored in a struct #710

Diff wrt to #257

As can be seen from the diff above, I needed to change the definition of the PODIO_DECLARE_LINK macro, and it now only deals with the registration to the CollectionBufferFactory and there is a separate macro to deal with the registration to the SIOBlockFactory. Otherwise, it becomes hard to only build the "central" part of a datamodel without any dependencies on an I/O backend. This could be lifted to #257 if we wanted.

[tmadlener]

From a technical point of view this is ready. I can successfully build the complete stack with this and key4hep/EDM4hep#373 without any changes to the stack, apart from a few cases where the deprecated Association headers were still in use and which should be fixed soon.

[tmadlener]

@peremato this should also address your concerns about having to discover these links for the Julia interface, I think. Now they will just be listed as a separate category in the YAML file, and you should have all the necessary information again to generate the necessary code, I think.

[tmadlener]

Now the layout in the files also uses a vector<podio::LinkData> instead of a vector<float> to store the weights of the links. This should make the switch from the pre-templated links to these ones completely transparent (apart from the changed typename).

[tmadlener]

Unless there are any more review comments, I will merge this later this week, to let EDM4hep move forward towards its 1.0 release.

[hegner]

Absolutely fine with me. If you want I can make a full review of the PR now

[tmadlener]

I am also waiting for a final confirmation from @peremato to make sure that the files that will be produced with this version are as expected on the Julia side.