Add `seed_duals` convenience and document how to seed Dual numbers

[ChrisRackauckas]

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Using the Dual implementation in ForwardDiff can be quite useful. This documents how to do it, and adds a seeding function seed_duals which seeds a unique partial for every component of a vector, much like is done internally (in fact, should it be used internally?).

[KristofferC]

FWIW, this increases the API surface of ForwardDiff considerably. Dual numbers are not even mentioned in the manual (except in the implementation details).

[pkofod]

FWIW, this increases the API surface of ForwardDiff considerably. Dual numbers are not even mentioned in the manual (except in the implementation details).

This is in the advanced section of the docs though.