Change gt4py imports in examples and tests

[romanc]

Description

This is a follow-up from #126. In this PR, we update the import paths in examples and tests to pull gt4py-related concepts from ndsl.dsl.gt4py.

Since we import from ndsl.dsl.gt4py in tests now, we actually don't need a separate test file for that and can thus safely delete tests/dsl/gt4py/test_gt4py_wrapper.py.

How Has This Been Tested?

I ran the notebooks locally. Tests are run by CI.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A

[FlorianDeconinck]

LGTM