Integrating vecmem into traccc and other projects

[stephenswat]

We've gotten quite a lot of nice code into this repository in the past few weeks, and during the ACTS parallelization meeting last Friday we discussed a little about starting to integrate vecmem into traccc and other co-processor-aware code. Now, I'd like to get a little bit of discussion started about what that means for us and what we still need to develop in order to allow that to happen. What are the crucial parts of this library that still need to be developed or improved before we can start integrating it into other projects?

The first one that comes to mind for me is good support for jagged arrays; they are used a lot in tracking, it seems. I'll start another discussion thread in a moment because it's a rather complicated topic. Is there anything else that is urgently needed? And what should our roadmap look like?