Warning is thrown when stacking on differing domains

Closes #1111
the perf here is quite bad: `O(d*n^2)` (and its not even doing anything, just showing a warning)
Here is an `O(dn)` solution
```
var datasetKeySets = this._datasetKeysInOrder.map((datasetKey) => {
  var dataset = ...
  var domainKeys ...
  return d3.set(domainKeys)
});
var unionSet = // merge all the sets
if keySets.some((s) => s.length !== unionSet.length
  warn
```