[singhaa] Adding tests

[adr1ja]

• Added checks in diffusion2d.py to ensure input parameters are valid numbers (floats).
• Unit Tests: Created tests in tests/unit/ to verify individual functions like initialize_domain and initialize_physical_parameters.
• Integration Tests: Created tests in tests/integration/ to verify that functions work correctly when called together.
• Coverage: Generated a code coverage report (coverage-report.pdf) showing ~61% coverage.
• Automation: Added tox.toml and requirements.txt to automate testing.