Implement the dim.Dataset generic.
We currently use nrow, which uses dim under the hood, so writing a generic for dim makes more sense here.
This enables all of dim, nrow and ncol to work.
[ ] Implement dim.Dataset generic
This can just call dim(x$env$data), so eventually dim.data.table is called
Implement the
dim.Datasetgeneric. We currently usenrow, which usesdimunder the hood, so writing a generic fordimmakes more sense here. This enables all ofdim,nrowandncolto work.dim.Datasetgenericdim(x$env$data), so eventuallydim.data.tableis callednrowdefinitions inDataset.R