Allow non-array values to be added to Crossfilter

If you pass an `object` to the `add` method then it will fail silently. Crossfilter is expecting an array. There are a few solutions that I thought of:

 * Notify the developer if they attempt to add an `object`;
 * Change the name of the `add` method to something that implies pluralisation, such as `addRecords`/`addMany`;
 * Transform the value into an `array` if it isn't already;

I chose the last one, but happy to discuss alternatives.