Fix bug in add_plot_id.

The text function requires the coordinates to be doubles.

src/@deproj/add_plot_id.m

@@ -7,8 +7,9 @@ function hts = add_plot_id( obj, ax )
     
     for i = 1 :  n_objects
         o = epicells( i );
+        center = double( o.center );
         hts(i ) = text( ax,  ...
-            o.center(1), o.center(2), o.center(3) + 0.5, ...
+            center(1), center(2), center(3) + 0.5, ...
             num2str( o.id ), ...
             'HorizontalAlignment', 'center', ...
             'VerticalAlignment', 'middle' );