WIP:  lambda grouper based on sliding window

number of args in function determines window width
df.groupby() `offset` kwd determines window offset
relative to current element.

df.groupby() `fill` kwd determines the value provided
for values for "phantom" locations at edges.

grouper function get a `_` variable injected into it's
context, intialized to 0, and containing the value returned
from the last call to the function. serves as an "accumalator"
that you can use to return the previous/new grouping label

Not optimized, just experimenting with the API.