Different header height using grouped header

I'm having an issue when trying to set a different header height using the grouped header plugin: The header comes twice as big in relation to the rest of the rows because it takes the default row height I'm setting. So I would like to set a height for the header and a height for the rest of the rows.

I tried to use the [Dynamic Grid Properties](https://github.com/fin-hypergrid/core/wiki/Dynamic-Grid-Properties) like this example

```
Object.defineProperty(grid.properties, 'backgroundColor', { get: function() {
var cell = this.gridCell;
return cell && (cell.x ^ cell.y) & 1 ? 'grey' : 'white';
}});
```

but this.gridCell comes undefined, so I don´t have a way to compare and differentiate the header cells from the rest of the cells.

Thank you in advance!

Start with the grouped header plugin and the Dynamic Grid Properties example linked in the issue, then reproduce the different header and row heights described. Trace why gridCell is undefined in that context and document or test a supported way to distinguish header cells; the issue is done when separate heights work without changing the remaining rows.

Written by the indexing model from the issue text.