Add region(::Plan) for accessing transformed region

[devmotion]

Currently, there is no official and consistent way for accessing the transformed region of a FFT plan. For many plans, plan.region works but e.g. it fails for ScaledPlan. This is problematic when one tries to write generic implementations, e.g., of derivatives (see, e.g., JuliaMath/FFTW.jl#182).

This PR adds a function region(p::Plan) that can be used to access the region of a plan. It falls back to p.region but can be specialized, e.g., for ScaledPlan.