feat(vtk): improve export at vertices (#844)

* Use bilinear interpolation to calculate the z of cell vertices (this is more appropriate than simple averaging when the grid is not regular)
* Ignore inactive cells when calculating the z of cell vertices (until now, they were not ignored, potentially yielding wrong values at the boundary of inactive cells)
* Use trilinear interpolation to calculate array values at cell vertices (this is more appropriate than simple averaging when the grid is not regular; furthermore, until now, everything was shifted downward with an awkward repetition of values at the top and bottom of the first layer)
* Improve performance by using scipy functions and avoiding loops (until now, interpolation was done by looping through all the cells in Python)
* Allow for directly passing array values at vertices to functions add_array() and add_vector() of the Vtk class
* New option "position" in the function postprocessing.get_specific_discharge() for calculating the values at "centers" (default), "faces" or "vertices"
* Cast delc, delr, top and botm of the grid to float64 (float32 was yielding significant round-off errors when rotating grid coordinates)

README.md

@@ -209,6 +209,8 @@ Additional dependencies to use optional FloPy helper methods are listed below.
 | `Raster()` in `flopy.utils.Raster`                                                   | **rasterio**, **affine**, and **scipy**            |
 | `Raster().sample_polygon()` in `flopy.utils.Raster`                                  | **shapely**                                        |
 | `Raster().crop()` in `flopy.utils.Raster`                                            | **shapely**                                        |
+| `._zverts_smooth()` in `flopy.discretization.structuredgrid` `StructuredGrid` class  | **scipy.interpolate**                              |
+| `.array_at_verts()` in `flopy.discretization.structuredgrid` `StructuredGrid` class  | **scipy.interpolate**                              |
 
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