Table header item used within models-table/table-header.

Each grouped-header should represent one item from ModelsTable.groupedHeaders.

Usage example:

const groupedHeaders = [
  [{title: 'BigTitle', colspan: 5}],
  [{title: 'SubTitle1', colspan: 2}, {title: 'SubTitle2', colspan: 3}]
];

<ModelsTable
  @columns={{columns}}
  @data={{data}} as |MT|>
  <MT.Table as |Table|>
    <Table.Header as |Header|>
      {{#each groupedHeaders as |groupedHeader|}}
        <Header.GroupedHeader @groupedHeader={{groupedHeader}} as |GroupedHeader|>
          {{#each GroupedHeader.groupedHeader as |cell|}}
            <th colspan={{cell.colspan}} rowspan={{cell.rowspan}}>{{cell.title}}</th>
          {{/each}}
        </Header.GroupedHeader>
      {{/each}}
    </Table.Header>
  </MT.Table>
</ModelsTable>