Apply some important fixes

[yguclu]

• Use pyccel >= 2.2.3: this allows compiling all kernels with C
• Require h5py >= 3.16 to allow for standard install with setuptools >= 81.0
• Require numpy >= 2.1 to support Python >= 3.10
• Use pytest.toml instead of pytest.ini for Pytest configuration (requires pytest >= 9.0)
• Move coverage configuration from pyproject.toml to psydac/pytest.toml
• Don't run postprocessing unit tests with pytest-xdist because h5py is not thread-safe: add Pytest mark @pytest.mark.xdist_group('h5py') to all unit tests in psydac/api/tests/test_postprocessing.py
• Return error code on failure of the psydac test command; this was not correctly returned before, hence the CI could not detect when the unit tests were failing in a subprocess
• Return error code on failure of the psydac compile command; this fix does not directly affect the CI, but it is included for correctness.
• Check correct reporting of failure for psydac test command in CI testing
• Use correct configuration file in coverage CI tests
• For clarity, rename temporary folder where all CI tests are run: pytest -> scratch
• Clean up .gitignore; specifically, stop ignoring files which don't exist anymore

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +4.94%	✅ 28.57%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (16fb9d3)	31788	19587	61.62%
Head commit (637a38f)	98952 (+67164)	65858 (+46271)	66.56% (+4.94%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#579)	35	10	28.57%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[campospinto]

This PR has a lot of improvements, thanks ! I can only do a shallow review but it seems good to me. I just have one question mostly to clarify a point.