The container expects that the particle exposes a method `as_tuple` that
converts the particle object to an std::tuple.

The container no longer hides the underlying container value_type. It
provides a particle_t alias to the particle type.

FTree will deduce its particle type from the container in two steps:
  - if the container exposes a particle_t type, it is used
  - otherwise, the value_type alias of the container is used