Hide FFT implementation details

[jngrad]

Fixes #4945

Description of changes:

• fully encapsulate the historic FFT implementation
  • electrostatics and magnetostatics now share the same FFT operations
  • FFT operations and mesh buffers are now better documented
  • all temporary mesh buffers are now private data members
• provide a type-erased API for the FFT backend
  • the P3M algorithm no longer has visibility of the FFT library
  • new FFT libraries can be used in place of the historic FFT implementation
• bugfixes:
  • the P3M algorithm no longer leaks memory when FFT plans are discarded
  • the FFT implementation now keeps the MPI environment alive until all FFT plans have been destroyed
    • avoids a race condition, since the FFT plan destructor need a MPI communicator

[jngrad]

@RudolfWeeber I realized just now we could improve separation of concerns in the P3M base struct further by moving the dipolar flag to the FFT class. Now it's ready to be merged.

@rodrigoacb We have disentangled the FFT logic from the P3M logic. There is still a small overlap due to how P3M communication buffers need to know about the FFT, but everything is encapsulated into the FFTBackendLegacy class. Alternative FFT libraries can be introduced via the CMake FetchContent mechanism and the FFTBackend pure virtual base class.