Transform interface and example Power transformation

[glennmoy]

PoC for the package interface (modelled off Impute.jl) and a prototype transform.

[rofinn]

I feel like maybe this package should be called Transforms.jl so that we can just have a Transform type?

[glennmoy]

I feel like maybe this package should be called Transforms.jl so that we can just have a Transform type?

Yeah perhaps that's better

Edit: I've now changed the name of the package and updated all references in the code.

[rofinn]

Makes sense to me, though I'm obviously biased. I think some more extensive testing and sample transforms would help. We may also need to do some iteration on the kwargs and function names at a later time.