Data parameter for grids

[gskinner]

It would be nice to add a data parameter for grids, so that those values can be set externally. One consideration is what to do with the default value – including the full list of items seems excessive, but it would be valuable to include at least one item to make it easy to reference the fields. This is complicated by the current requirement to define parameters in the constructor instead of during serialization.

[gskinner]

This is partially complete, but currently includes an empty default. Still needs updating to include the first item as the default. Requires testing.