Reconsider class for ACF lag column

[mitchelloharawild]

From #70

Use a difftime to represent lags instead of the cf_lag class.
Store the interval elsewhere for plot breaks.

[mitchelloharawild]

@earowang
I don't think difftime can be used because it doesn't support irregular units such as months. Does this sound reasonable?

[AEBilgrau]

Hi, I was about to make an issue in respect. I lack some functionality for either type conversion/coercion or cf_lag handling in general. The current class is a bit cumbersome in the downsteam analysis and e.g. wanting to use other plotting libraries than ggplot2. What are your thoughts on this?

[mitchelloharawild]

I have ideas for a new temporal vctrs object for handling temporal aggregation (tidyverts/fabletools#59).
This new temporal class will also support origin-less time offsets which ACF will use.
It will have also have scales methods so it should work with ggplot2.

[mitchelloharawild]

Wait for https://github.com/mitchelloharawild/moment to provide origin-less time structures