Custom Transform

[glennmoy]

As mentioned in #5 and #6, a CustomTransform (or some other name) would be useful when one wants to apply some custom function to their data.

This would be quite useful since we then wouldn't have to write a new Transform for each of these.

TODO: MWE

[nicoleepp]

Something like

"""
    CustomTransform <: Transform

Apply the function passed in to CustomTransform as the transform.
"""
struct CustomTransform <: Transform
    transform_function::Function
    args
end

function _apply!(x::AbstractArray, CT::CustomTransform; kwargs...)
    return CT.transform_function(x, ct.args...; kwargs...)
end