Add type hinting

[djmhunt]

Type hinting, as described in PEP 484 should be added to as many parts of the code as possible.

A reasonable way forward would be to:

1. Convert the Python 2.7 type hinting to the Python 3 style
2. Add it to functions that are not model, experiment or fitting method specific
3. Add it to areas that already have unit tests
4. Add it to remaining classes that are not model, experiment or fitting method specific
5. Add it to all remaining code

Use of an IDE, such as PyCharm that uses type hinting will simplify the task.