Remove the need for calling data in cube.data.rechunk

[HGWright]

When rechunking a cube, but we always want to rechunk the data. This makes the need to call data superfluous. It also requires an extra step in Iris understanding that would make it more approachable if removed.

But we are also not keep to remove it from cube.data as that would require changes to existing workflows and possibly confuse people that understood the previous behaviour.

Taking that into account, and in an effort to dodge future problems, we think that a thin wrapper around the existing behaviour so that both versions exist without the need for duplicated code.

This is certainly not set in stone and so we welcome thoughts and or suggestions about this.