Apply changes to geertsma branch

[gabrielserrao]

Implemented geertsma, both for disc and nucleous of strain

[prisae]

Thanks Gabriel, this is great! I will have a look over the weekend or next week.

Currently all tests fail, primarily due to flak8 (code style). I can sort that out, but if you want, you can also tackle it. Locally, just run make flake8, and it will lint your code and give you feedback.

You can also run locally make pytest, to see if the actual tests pass.

[prisae]

Also, any code added to dageo (not to the examples) will have to be covered by tests. We can work on that together.