Distinguish NA (missing) from NA (accumulated)

[sbfnk]

Enabling this would, I think, require some sort of that marks dates explicitly as missing vs. NA.

I think this would be my preferred option as it would be more general but I also think it can be addressed in its own review as it would be a superset of this PR.

My thought on how that would work is to have a new variable (accumulate) that indicates which days should be summed.

Originally posted by @seabbs in #534 (review)

[sbfnk]

My thought on how that would work is to have a new variable (accumulate) that indicates which days should be summed.

Another option would be to distinguish between explicit (date exists in the data, value is NA meaning missing) vs. implicit (date doesn't exist in the data meaning accumulate) NAs which might be easier preprocessing if potentially easier to inadvertently get wrong.

[seabbs]

yeah potentially but also think that could be a bit dangerous. I would instead suggest making a helper function that maps from that structure to the less dangerous explicit version for those that clearly want that.

[seabbs]

and if going that way I'd suggest that becomes a dependent issue