Describe the proposal
We often need to do the same set of operations over all MultiFabs and ParticleContainers, but C++ doesn't provide a capability to iterate over all of the member variables in a class, so we have to hard code them. However, can instead dynamically allocate MultiFabs and keep track of a list of all of the MultiFabs ourselves.

WarpX does this with its MultiFabRegister. This creates an object that behaves like a dictionary (e.g., in Python), where we can access (or create) the MultiFabs with a string, and also loop over all of them with an iterator.

We also need to do this with particle containers, since we may have many particle containers, and we don't want to hard code everywhere we need to redistribute particles, write them to disk, etc., for each particle container individually.

If we need to use the MultiFabs inside a amrex::ParallelFor kernel, we just get a reference to the MultiFab from the MultiFabManager while outside the ParallelFor, and from then on, use it as we normally would.

Describe alternatives you've considered
Keep as-is. If we need to loop over all MultiFabs or ParticleContainers, we do it manually.

Additional context
https://github.com/ECP-WarpX/WarpX/blob/fb9e7dd1a2946c1f293cd25f2061d78cf5d1d71f/Source/ablastr/fields/MultiFabRegister.H#L156