Add note about weighting and codebook

[1beb]

Codebooks are designed to work with whole numbers. A weighted dataset present a count as a decimal and this breaks the presentation of the codebook significantly.

We should warn users of this behavior, and denote the fix in examples:

weight(ds) <- NULL

Datasets must be unweighted and this needs to be made clear for the user. Consider adding an in-code warning or stop(). We can't "do it for them" because we don't want to edit the state of a dataset on behafl of a user (let's say they are generating a codebook for a client but the client is also looking at these data in crunch).