[oezsoyik] Adding Tests

[ozsoyisik]

• After I added assertions in diffusion2d.py, I first got Assertion Errors because of the integer default values. I corrected the default values.
• The test coverage becomes 80% with the implementation of unit and integration tests which can be found in coverage-report.pdf.
• By calling "tox" command, the tests can be run in a separate environment using pytest.