Add functions pvgridder.extract_layer() and pvgridder.slice_vertical()

[keurfonluu]

• Added: function pvgridder.extract_layer() to extract a specific layer given its ID from a mesh generated by pvgridder.MeshStack3D().
• Added: function pvgridder.slice_vertical() to extract a vertical slice given a 2D line/polyline from a 3D mesh.

Reminders:

• Run invoke ruff to make sure the code follows the style guide,
• Add tests for new features or tests that would have caught the bug that you're fixing,
• Write detailed docstrings for all functions, classes and/or methods,
• If adding new functionality, unit test it and add it to the documentation.