fixed interpol with null index value

WindStressPRM/Utils/Matrix.cs

@@ -25,7 +25,7 @@ namespace WindStressPRM
         /// get index of cell for coordinate
         /// </summary>
         /// <param name="coordinate"></param>
-        /// <returns></returns>
+        /// <returns>null if it doesn't exist</returns>
         public Index ProjectionToCell(Coordinate coordinate)
         {
             double rwDiff = (coordinate.X - Origin.X) / Size.Width;
@@ -35,7 +35,7 @@ namespace WindStressPRM
 
             if (iRow < 0 || iCol < 0 || iRow >= RowsCount() || iCol >= ColumnCount())
             {
-                throw new Exception("projectionToCell method trying to find uncorrect index");
+                return null;
             }
             return new Index(iRow, iCol);            
         }

WindStressPRM/WindStressPRM.cs

@@ -319,6 +319,10 @@ namespace WindStressPRM
             const bool normalX = true;// true - East, false West
             const bool normalY = false;// true - North, false South
             Index rc = matrix.ProjectionToCell(coords);
+            if (rc == null) {
+                // return NaN, because there is no correct index
+                return Double.NaN;
+            }
             Coordinate center = matrix.CellToProjection(rc);
             double xDiff = coords.X - center.X;
             double yDiff = coords.Y - center.Y;