User interface for accessing W operator components

[isaacsas]

Per Slack discussion with @ChrisRackauckas, opening an issue about the need to have an interface for users to get at the W operator components. In my case I have a periodic reaction-diffusion system, so for many methods the operator should have the form W = I - const*L, where L is the constant Laplacian operator of the system. If I want to use FFT's to invert W, it seems like I need to have a way to get at the current value of the constant when writing an FFT-based linear solver.