[skip ci] Make sure that intersection between cells is not planar

...t/poisson/facedir-facemod-variation/generate_meshfiles.py

...oisson/facedir-facemod-variation/grid_unstructured_00.msh

...oisson/facedir-facemod-variation/grid_unstructured_01.msh

...oisson/facedir-facemod-variation/grid_unstructured_02.msh

...oisson/facedir-facemod-variation/grid_unstructured_03.msh

...oisson/facedir-facemod-variation/grid_unstructured_04.msh

...oisson/facedir-facemod-variation/grid_unstructured_05.msh

...oisson/facedir-facemod-variation/grid_unstructured_06.msh

...oisson/facedir-facemod-variation/grid_unstructured_07.msh

...oisson/facedir-facemod-variation/grid_unstructured_08.msh

...oisson/facedir-facemod-variation/grid_unstructured_09.msh

...oisson/facedir-facemod-variation/grid_unstructured_10.msh

...oisson/facedir-facemod-variation/grid_unstructured_11.msh

...oisson/facedir-facemod-variation/grid_unstructured_12.msh

...oisson/facedir-facemod-variation/grid_unstructured_13.msh

...oisson/facedir-facemod-variation/grid_unstructured_14.msh

...oisson/facedir-facemod-variation/grid_unstructured_15.msh

...oisson/facedir-facemod-variation/grid_unstructured_16.msh

...oisson/facedir-facemod-variation/grid_unstructured_17.msh

...oisson/facedir-facemod-variation/grid_unstructured_18.msh