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