Add noise gating

[dstansby]

Craig De Forest has written a python implementation of his noise gating algorithm here: https://github.com/drzowie/image-noise-gate. It would be good to incorporate this into sunkit-image, either by directly including the code (would need to check on licensing though, as it has no license yet as far as I can see) or wrapping it so it can be easily applied to sunpy maps.