v0.7.0: tweak tag generation (#279)
* tweak tag generation
This does 3 related things
1. Re-enables void tags by providing `nothing` as function argument
2. Allows opt out of tag checking via `Val{false}()` argument.
3. Simplifies the Hessian to use the same tag (which is useful for my own purposes of computing Riemannian metrics, but also makes the code a bit cleaner).
* add docs
* derivative fixes
* add tests for tag checks
* add tag docs