Deprecate registerAlgorithm in favour of AlgorithmFactory.subscribe

This is more in line with how the new functions work and how the C++
side works too. Refs #970
Pass python object by reference to const rather than value. Refs #970
Allow python functions to override catgeories.

Also adds simple Python Jacobian test.
Refs #970
Flag new directory keys to ConfigService so it turns them absolute

Refs #970
Merge branch 'feature/970_python_fitfunctions' into develop