Add visit-like pattern for RecordComponent

[franzpoeschel]

Close #1372

TODO:

• Once we agree on a design, add some further examples

This adds:

1. std::variant<...> aliases for openPMD datatypes
2. A RecordComponent::visit(...) call that instantiates the passed Functor Template Object with the current type of the RecordComponent
3. Using 1. and 2., a RecordComponent::loadChunkVariant() call that returns a std::variant<...> of shared_ptr types

[ax3l]

Very nice!