Fix export to vtk

[mkondratyev85]

Description

A small change that solves issue #641 that was causing a bug while exporting a model to vtk via Pyvista.
The Pyvista receives a flatten array but the gempy passed 3D array of lithology to set values to mesh cells.

As only a single line was changed I don't know if I have to check boxes in the checklists below, so I leave them unchecked for now.

Checklist

• My code follows the PEP 8 style guidelines.
• My code uses type hinting for function and method arguments and return values.
• My code contains descriptive and helpful docstrings
  which are formatted per the Google Python Style Guidelines.
• I have created tests which entirely cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests)
  or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New and existing tests pass locally with my changes.

[AlexanderJuestel]

@mkondratyev85 thanks for opening the PR, I still have to reproduce the issue in the first place. If your fix works, which I am sure it does, I would merge it to my dev branch where I am working on #640. Would that be okay?

[AlexanderJuestel]

I can confirm that the proposed change will fix issue #641

[mkondratyev85]

Hi @AlexanderJuestel! Thank you so much for the help!