Ag Grid adaptor doesn't handle runtime reset of columns

[heswell]

The ViewportRowModelDataSource adapts a VuuDataSource to the Ag Grid equivalent. It used some mapping structures to map data between a Vuu row representation and an Ag Grid row representation.

There is a built-in and undocumented assumption that the VuuDataSource provided to constructor will already have columns defined. This might not be the case, though, client may defer definition of columns.