Test Korakianitis mixed model

[crangelsmith]

Adding test for Korakianitis model, adding instantiation and numerical checks.

Things to review:

• Is this a good representation of the model? It could be configured better to represent typical use.
• Is the numerical checks enough? See line 64.

[LevanBokeria]

Hi @MaxBalmus

• I have updated the KorakianitisMixedModel test to now load a json file containing expected values instead of defining them inside the test code.
• For clarity, I have removed the Naghavi model tests from this branch, and moved those to a separate branch Test Naghavi model #24.
• I think the KorakianitisMixedModel test can be improved by (i) adding comments and explanations, (ii) improving the tests to be more comprehensive. I think for now, its worth having this test run as is so that we have at least something running while we improve the codebase in other directions (such as pre-commit hooks, etc). But we should go back to KorakianitisMixedModel tests at some point.
• If you have a particular pointer to a tutorial file that would help me run KorakianitisMixedModel and understand it more, that would help me develop better tests later.

I'll now mark this review as done, and if you concur feel free to merge this with dev.

[MaxBalmus]

I had a look and everything looks good to me.