add an example for exporting model to VTK

[yangjiandendi]

Description

added an example for exporting GemPy model to VTK model and getting the vertices of model mesh in Example 01.

Relates to <#922>

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).
• [x] New and existing tests pass locally with my changes.

[Leguark]

Hi yangjiandi. Thanks for the pull request, this can be a nice example for other people. However, to add it to the documentation it should be converted to a .py file with similar syntax as the other examples. This makes possible to build sphinx documentation. If you convert it into a .py file I can continue the review.

Cheers