Reduction operations for more containers, operator-overloaded classes #75
Description
Some of these are going to end up in libMesh and MetaPhysicL, but some will be in TIMPI and they'll all be based on the TIMPI APIs, so here's probably the best place to track the issue.
We have a ton of classes, from std::pair to libMesh::TypeTensor, which are often (or always) carrying numeric contents, but even though we've now got communication capabilities (via StandardType specializations) for these classes, we still generally don't have reduction capabilities (via OpFunction specializations) for them.
@andrsd needs some of this for a FY22 NEAMS T/H task, so I've got funding from there to do it right, not just patch up the API one specialization at a time.
This isn't as urgent as a couple other tasks I'm on right now, but hopefully I'll be starting on it in a couple weeks.