notebook for fitting of stretched exponential

[jmborr]

Description of work.

To test:

Fixes #49

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the [coding standards]?
• Are the unit tests small and test the class in isolation?
• If there are changes in HISTORY.rst then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

Does everything look good? Mark the review as Approve. A member of @qef/gatekeepers will take care of it.