Rework and fix the implementation of MinMeanCycle (#179)

- Fix the handling of the cycle means.
- Many implementation improvements:
- More efficient data storage for the strongly connected
components.
- Better handling of BFS queues.
- Merge consecutive BFS searches (perform two BFS searches
instead of three).

This version is about two times faster on average and an order of
magnitude faster if there are a lot of strongly connected components.

@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