Vis: Fix bug with vtk writing of non-square wave surface

When a rectangular SeaState grid was used, the connection list in the vtk file was incorrectly generated due to an indexing error.  The list would contain points beyond the total number of points in the point list.

For a 5x3 SeaState grid (Nx=3, Ny=2) there are a total of 15 points (0 to 14).
The connection list was being incorrectly generated as
```
        <DataArray type="Int32" Name="connectivity" format="ascii">
      0      1      3
      1      4      3
      1      2      4
      2      5      4
      5      6      8
      6      9      8
      6      7      9
      7     10      9
     10     11     13
     11     14     13
     11     12     14
     12     15     14
     15     16     18
     16     19     18
     16     17     19
     17     20     19
```

The connection list should be
```
        <DataArray type="Int32" Name="connectivity" format="ascii">
      0      1      3
      1      4      3
      1      2      4
      2      5      4
      3      4      6
      4      7      6
      4      5      7
      5      8      7
      6      7      9
      7     10      9
      7      8     10
      8     11     10
      9     10     12
     10     13     12
     10     11     13
     11     14     13
```

modules/openfast-library/src/FAST_Subs.f90

@@ -8686,7 +8686,7 @@ SUBROUTINE WrVTK_WaveElevVisGrid(t_global, p_FAST, y_FAST, SeaSt)
 
    do ix=1,p_FAST%VTK_surface%NWaveElevPts(1)-1
       do iy=1,p_FAST%VTK_surface%NWaveElevPts(2)-1
-         n = p_FAST%VTK_surface%NWaveElevPts(1)*(ix-1)+iy - 1 ! points start at 0
+         n = p_FAST%VTK_surface%NWaveElevPts(2)*(ix-1)+iy - 1 ! points start at 0
 
          WRITE(Un,'(3(i7))') n,   n+1,                                    n+p_FAST%VTK_surface%NWaveElevPts(2)
          WRITE(Un,'(3(i7))') n+1, n+1+p_FAST%VTK_surface%NWaveElevPts(2), n+p_FAST%VTK_surface%NWaveElevPts(2)