Add Binomial Heap (TheAlgorithms#1146)

* Binomial Heap

Implementation of Binomial Heap. Reference: Advanced Data Structures, Peter Brass

* Update binomial_heap.py

* Update binomial_heap.py

* Update binomial_heap.py

- Fuller documentation of binomial heap
- Update unit tests
- Replace printing method by overwriting __str__()

* Update binomial_heap.py

- Added more tests
- Added to the documentation
- Stylistic editing
- mergeHeaps now also returns a reference to the merged heap
- added a preOrder function that returns a list with the preorder of the heap

* Update binomial_heap.py

Changed the unit tests structure

* Turned the tests into doctests