[dardama] Adding tests

[megha-darda]

This pull request adds a complete testing setup for the 2D diffusion solver.

What was added:

• Assertions in the solver to enforce float input types for domain and physical parameters
• Unit tests for below methods in SolveDiffusion2D:
  • initialize_domain
  • initialize_physical_parameters
  • set_initial_condition
• Integration tests to verify that multiple methods work correctly together:
  • test_initialize_physical_parameters
  • test_set_initial_condition
• Test coverage report generated using coverage
• Tox configuration to automate running unit tests, integration tests, and coverage in isolated environments

Checks:
I tested if the test cases failed if I introduced breaking changes:

1. When i changed self.w = w to self.w = h in initialize_domain,
   test_initialize_domain unit test and both the integration tests failed as expected

  FAILED tests/integration/test_diffusion2d.py::test_initialize_physical_parameters - assert 7.5 == 12.0
  FAILED tests/integration/test_diffusion2d.py::test_set_initial_condition - assert 5.0 == 10.0
  FAILED tests/unit/test_diffusion2d_functions.py::test_initialize_domain - assert 7.5 == 12.0

2. When i changed self.T_cold = T_cold to self.T_cold = T_hot in initialize_physical_parameters,
   test_initialize_physical_parameters unit test and again both the integration tests failed as expected

FAILED tests/integration/test_diffusion2d.py::test_initialize_physical_parameters - assert 900.0 == 250.0
FAILED tests/integration/test_diffusion2d.py::test_set_initial_condition - assert 700.0 == 300.0
FAILED tests/unit/test_diffusion2d_functions.py::test_initialize_physical_parameters - assert 900.0 == 250.0

3. When i changed if p2 < r2 to if p2 > r2 in set_initial_condition,
   test_set_initial_condition unit test and only the test_set_initial_condition integration test failed as expected

FAILED tests/integration/test_diffusion2d.py::test_set_initial_condition - assert False
FAILED tests/unit/test_diffusion2d_functions.py::test_set_initial_condition - assert False