Add d3.movingAverage as a native d3-array function

[rdmurphy]

I've borrowed the moving average function found in this notebook a few times now, and having just done it again 😁 it made me wonder if it belongs in d3-array proper? I feel like it has a similar vibe to d3.cumsum.

The only pause I have is whether there is an issue with its definition of what constitutes the "moving average." I think I know how to modify to be center-based ((N - 1) / 2 values on either side of V vs. V and N - 1 previous values), but don't know if alternative definitions would/should be on the d3-array namespace under different names or as options to pass to a d3.movingAverage itself.

Either way, thank you!

[Fil]

this or d3.blur would be great yes.

Closing as a duplicate of #56 (in particular #56 (comment)) & #151