Interpolate generic initializer

[romanroibu]

Use generic initializer to create Interpolate objects. This way, there is no need to typecast inside the apply block, so the code becomes cleaner.

let colorChange = Interpolate(from: UIColor.whiteColor(), to: UIColor.redColor(), apply: { [weak self] (color) in
    self?.view.backgroundColor = color
})

Also, this guarantees that the type of from parameter is the same as the type of to, so there is no way to write code like this:

let change = Interpolate(from: UIColor.whiteColor(), to: 10.0, apply: { [weak self] (result) in
    // This will cause a fatal error in calculateDiff
})

[marmelroy]

Thanks @romanroibu ! Big improvement! Don't know why I didn't think about it...