Use Holy traits for variable transformations

[cgarling]

Using 1, 0, -1 to indicate different transformation types, but this is runtime information that needs to be checked with if statements. Could use types (PositiveTransformation, NoTransformation, NegativeTransformation), and then define methods that operate on those types which should move the computation to compile time. Haven't tried this before but this webpage has some info on how to do this.

[cgarling]

Alternatively we could look into using TransformVariables.jl, but it seems a bit like overkill, probably don't need all the features it provides.