Downstream packages that use `stat_contour()` need to add isoband to Suggests

[paleolimbot]

Just a note from going through these revdeps that several packages fail as a result of needing the isoband package, which is used for stat_contour() and friends. This is new after #3439.

Packages that fail are bayesAB, fgeo.plot, and gganimate.

[clauswilke]

Should we list isoband under Imports? I would think of making contour plots as a basic feature that should work out of the box after installing ggplot2. Similarly, we also import mgcv.

The isoband package does not pull in any additional dependencies, as I designed it specifically that way. It only depends on Rcpp, grid, and utils, which are already either direct or indirect dependencies of ggplot2.

[thomasp85]

If we rely on it for something that were dependency free before I think we need to import it to avoid breaking code

[yutannihilation]

+1 for adding isoband to Imports.

[paleolimbot]

I agree...I was waiting for the last PR I did to get reviewed before starting a new one, but could easily tackle this one later today.

[lock]

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