init commit for BinaryHeap package

Nothing here yet
The implementation of Heap
Fix remove and get of non-existent id
Tests for BinaryHeap
Rename heapify to downHeap to keep it consistent with upHeap

Most text-books use "heapify" but in one text-book I remember the use of
"downHeap", so I didn't make it up.
A step to MaxMinHeap
Do OOP right
Fix _upHeap and fix oop once again