Feature/d2 strings as constants

[ciaradunks]

Fix #806

Changes proposed in this pull request:

• Define strings as constants in D2 module
• Import and use constants from D2 module in test_D2_model_constraints

The following steps were realized, as well (if applies):

• [x] Use in-line comments to explain your code
• :x: Write docstrings to your code (example docstring)
• :x: For new functionalities: Explain in readthedocs
• :x: Write test(s) for your new patch of code (pytests, assertion debug messages)
• [x] Update the CHANGELOG.md
• [x] Apply black (black . --exclude docs/)
• [x] Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}

[ciaradunks]

@smartie2076 I've applied your suggested changes (you have to either import the constants at the top or refer to them as D2.constant_name). I've changed it now to be D2.constant_name so have a look if you prefer it like this. Other than that, this PR is ready to go.

[ciaradunks]

@smartie2076 this should be ready to merge now