TransformVariables API change

[tpapp]

This is just a head-up that there will be a minor API change in TransformVariables (tpapp/TransformVariables.jl#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.