Change default boundary value to "none" for dask map_overlap

I’d like to propose changing the default value of the boundary keyword argument in map_overlap. The current default is "reflect", and I’d like it to be boundary="none" instead.

Reasons why this would be useful:

• It’s much faster. It roughly halves the number of tasks, and the difference in compute time is significant
• There’s less of a difference between results from a numpy array, vs the same data in a dask array (numpy doesn’t do extra padding around the edges by default, so when we do this with dask, it’s not equivalent anymore)
• Users rely on default values to choose something sensible, and I think fast performance should be weighted more highly as a sensible priority.
• Many inner functions already handle edges themselves (eg: scipy.ndimage.gaussian_filter, among many others), in which case it is redundant to handle the edges twice.

dask-image already uses boundary="none" wherever possible. We also see users concerned about performance end up doing a bit of experimentation and ending up here too.

cc @jni @jakirkham

[jakirkham]

Ok so it doesn’t sound like this will cause issues. So maybe we should go ahead.

Just to make sure we haven’t missed anyone, would suggest a deprecation cycle. So first we warn if boundary is not specified and let users know the behavior will change (this will become "none" in some future release). Then in that future release (whatever version we picked), we make the change.

[GenevieveBuckley]

Update, we said we’d make the change in version 2022.03.0

So in late February, we need to make a second PR, changing the default value of the boundary kwarg to "none" (that’s not the python None, but a string with the word "none"). If you see the first PR linked above, there’s a comment telling you which function & line the change needs to be made on.