Update VTK writer

[ewquon]

Updates:

• Fix bug with incorrect spacings written out
  • Caused by issue with uniqueSorted function returning non-unique lists of x/y/z points
  • Fix: round sampling points to 3 decimal places
• Ensure correct structured VTK output with Fortran-ordering of points (x changes fastest, then y, then z)
  • See VTK File Formats -- "Dataset Format" section, p.3 (https://vtk.org/wp-content/uploads/2015/04/file-formats.pdf)
  • Note: the sorting will not work as expected given a rotated sampling box
• Add some error checking

This should address https://github.com/NREL/SOWFA-6/issues/30