TransformVariables API change

This is just a head-up that there will be a minor API change in TransformVariables (https://github.com/tpapp/TransformVariables.jl/pull/137) and it probably affects your package.

From version 0.9, which I will release soon, the method `inverse_eltype` needs to be defined for *types*, not *values*.

If you want your package to work with versions < 0.9 of TransformVariables.jl, you can just leave the current method in place and define one for types in addition. Otherwise, change to types, and restrict to `>= 0.9` in compat, we will tag a 1.0 in the near future after some other breaking changes.

Please do not hesitate to reach out if you need help, either in the PR above or in this issue.