Unify member names in heaps (#299)

The following renamings are made.

Public members:
- UnderFlowPriorityError -> PriorityUnderflowError
("underflow" is only one word)

Private members:
- bubble_up() -> bubbleUp()
- bubble_down() -> bubbleDown()
- second_child() -> secondChild()
- makeroot() -> makeRoot()
- relocate_last() -> relocateLast()
- data -> _data
- boxes -> _boxes

 @nodes

 label

 0

 1

 2

 3

 4

 5

 6

 7

 @arcs

 		label capacity

 0 	1 	0  	  16

 0 	2 	1 	  12

 0 	3 	2 	  20

 1 	2 	3 	  10

 1 	4 	4 	  10

 1 	5 	5 	  13

 2 	3 	6 	  10

 2 	4 	7 	  8

 2 	6 	8 	  8

 5 	3 	9 	  20

 3 	6 	10 	  25

 4 	7 	11 	  15

 5 	7 	12 	  15

 6 	7 	13 	  18

 @attributes

 source 0

 target 7