Add more tests for xarray grid shading

[seisman]

Description of proposed changes

This PR adds more tests for xarray grid shading.

Valid shading values are:

• boolean
• a constant intensity
• some modifiers to automatically derive an intensity grid from input grid
• an intensity grid with modifiers
• an intensity grid in the (-1,+1) range

This PR covers the first 4 cases only. Of course, it only works for GMT>=6.2.0

Address #618.

Reminders

• Run make format and make check 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.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.