do not assume 1st column holds row names

As of this commit, we no longer assume that the 1st column of
the input table specifies the names of the table's rows.  Instead,
we search by default for a column named "name".  The user can
change the name of this array by calling vtkHeatmapItem::
SetNameColumn(vtkStdString).  We fallback to using the 1st column
only if no such array can be found by name.

Change-Id: I2494ff21f3efa4a379dce82e159715dc023d8995
Merge topic 'heatmap_row_names' into master

729fe63f do not assume 1st column holds row names