535656: [Table] Cell containing multiple values are not displayed
correctly in generated document

Update PapyrusService.getGendocTable method to build differently a table
cell label when the cell contains a collection : instead of displaying a
label of the collection, display a collection of label

Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=535656

Change-Id: I7f558e4bfc47ba4d0f59382e4e82b95a14a70be4
Signed-off-by: ANNE HAUGOMMARD <anne.haugommard@atos.net>