Add SwarmPacks

[pdmullen]

This MR introduces the notion of a SwarmPack. It is a stripped down version of SparsePack, tuned towards particle swarms, to enable packing over MeshBlocks. I have preliminarily settled on a design wherein one can pack a SwarmPack via a std::vector<std::string> or the type-based variable prescription previously used in SparsePacks. SwarmPacks are unique from SparsePacks in that we must be able to handle both Real and integer types. In this MR, I do not allow SwarmPacks to contain mixed types, however, one can create a pack of Real swarm variables and a separate pack for int swarm variables. The design follows:

by std::vector<std::string> (where the template argument specifies that the data type):

std::vector<std::string> vars{swarm_position::x::name(),
                              swarm_position::y::name(),
                              swarm_position::z::name()};
static auto desc = MakeSwarmPackDescriptor<Real>(swarm_name, vars);
auto pack = desc.GetPack(md);

by type-based vars (where the data type is inferred from var-types):

static auto desc = MakeSwarmPackDescriptor<swarm_position::x,
                                           swarm_position::y,
                                           swarm_position::z>(swarm_name);
auto pack = desc.GetPack(md);

There are so many opportunities to further clean up things here... One could imagine (1) allowing mixed types in packs, (2) allowing for packing via Metadata or regex, or (4) combining SparsePacks and SwarmPacks, to name a few.

This MR adds the SwarmPack machinery to the particle_leapfrog test.

Finally, I'd note that this MR also transitions swarm_data to belong to meshblock_data rather than pmb. This likely has consequence when considering copies of mesh_data registers (e.g., in RK staging). Any undesired manifestations from this change should be uncovered as we continue to test downstream. For one, the current design requires that downstream users interface with particles via the base mesh_data register.

PR Checklist

• Code passes cpplint
• New features are documented.
• Adds a test for any bugs fixed. Adds tests for new features.
• Code is formatted
• Changes are summarized in CHANGELOG.md
• Change is breaking (API, behavior, ...)
  • Change is additionally added to CHANGELOG.md in the breaking section
  • PR is marked as breaking
  • Short summary API changes at the top of the PR (plus optionally with an automated update/fix script)
• CI has been triggered on Darwin for performance regression tests.
• Docs build
• (@lanl.gov employees) Update copyright on changed files
• Include caching for SwarmPacks (cache cleared whenever Swarm::setPoolMax is called)

[Yurlungur]

I am super happy to see this be added! Worth noting it does break downstream because you moved where swarms live to be in meshblock data (which is where they belong). Comments below.

[brryan]

After a brief first read, this looks great to me so far.

[lroberts36]

Nice, this LGTM. Some small non-blocking questions/comments below.

[Yurlungur]

I still have some minor nitpicks but nothing I would call blocking. Thanks for this cleanup @pdmullen it's really great to have swarm packs.

[pgrete]

FYI I'm happy with the discussion and don't plan a detailed review. So just go ahead an merge when everyone is happy.