Paginate by values of a dimension

[RickMoynihan]

Current draft of the spec doesn't let you pick which dimension to paginate on.

You could in principle paginate on two (or more) dimensions; though to keep things simple it's probably better to assume that you'll almost always want to paginate in one dimension only.

Should update the SPEC to allow for this, and provide a means of knowing the sizes of each dimension.

[RickMoynihan]

#24 lets you distinguish between the rows axis and the columns axis... Which whilst not strictly a mapping to x/y (an application could decide to render things rotated after all) it is a more convenient and concrete way to describe things and map to developer intuitions.

There are implications for this for the REST API, as it will need to provide a method for the application to specify which dimension to set as rows (i.e. paginate on).