Define behavior when a Grouper's value is undefined or not present

[apmcleod]

If an element cannot be grouped, instead of dropping it like here right now, we could also leave it in but with the old index. Or the behaviour could depend on a parameter similar to panda's .groupby(XY, dropna=False).

For example, let's say we're grouping by years but a couple of pieces don't have the year information in their metadata. Currently, those would "get lost" but we could give the choice to leave them in. The group name tuples would then have to be (previous_group, year), (previous_group, NaN). What do you think?

Originally posted by @johentsch in #1 (comment)

[apmcleod]

I like your idea, of allowing to include a NaN (or something) in the index in those cases, via a parameter.