Add option bottom_up to pvgridder.MeshStack2D() and pvgridder.MeshStack3D()

[keurfonluu]

• Added: new class option bottom_up to define stacking direction. This fixes a bug when negative scalar were passed in method add() due to a conflict with priority.

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.