FBasicParticleContainer's implementation of FAbrastractSendable
interface was buggy:
 - getSavedSize did not return the right result;
 - the extra attributes (type pack `OtherTypes`) were not saved.

The whole interface implementation has been moved to the base class
FVariadicParticleContainer and is now correct.

The undelying variadic_vector is modified to expose the integer_sequence
holding the indices for its types. This allows derived classes to use it
to act on all its data.