IPPL-2.1.0

Memory optimizations: Introduce the buffer factory interface, allowing MPI communication buffers to be allocated a single time. Redesign particle deletion to work in-place. Kokkos buffer resizing no longer unnecessarily preserves old data. Reduce host-space allocations for MPI requests.

Structural changes: Add the plasma mini-apps and remove or redesign redundant test programs. Introduce the FFT solver.