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

[andrew-platt]

Feature or improvement description

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

Related issue, if one exists
This should be backported to 3.5.4

Impacted areas of the software
Visualization of rectangular wave surfaces only.

Additional supporting information
Thanks to @luwang00 for finding this.

Test results, if applicable
This is not part of the regression tests (we don't output vtk in regression tests due to data size).

[luwang00]

@andrew-platt, thanks for putting this PR together.

[andrew-platt]

Thinking about this a bit more, there is no reason to fix it in 3.5.4 as only square wave grids are used there (hard coded). So we won't backport this.