ENH: throw exception when the date file has less than 2 dates

include/otbDates.h

@@ -93,7 +93,7 @@ typedef std::vector<SingleDate> DatesType;
  * Input string must be in the form of "YYYYMMDD ....."
  */
 DatesType GetDatesFromStringVector(std::vector<std::string> & list, bool check_order = true){
-
+std::cout << "GetDatesFromStringVector" << std::endl;
   DatesType dates;
   for (unsigned int i = 0 ; i < list.size() ; i++)
     {
@@ -119,6 +119,12 @@ DatesType GetDatesFromStringVector(std::vector<std::string> & list, bool check_o
         }
     }
 
+  // we need at least 2 dates !
+  if (dates.size()<2)
+    {
+      itkGenericExceptionMacro("Must be at least 2 dates to work");
+    }
+
   // Check dates ordering
   if (check_order)
     {