Rewrite minimum window filtering with bottleneck?

[angus-g]

xarray has ffill and bfill for filling NaNs by propagating values forward/backward (with an optional limit on the length of the propagation). This is almost exactly what we need to do for the minimum window functionality (where we essentially propagate values into NaNs at the ends of a sequence if there are fewer than a certain number of NaNs for a given particle).

If we can't use this directly, it's worth noting that these functions are implemented on top of bottleneck.push

[angus-g]

Code sample in #61 (comment) is probably a good starting place. Would need to check that this still has the same semantics.