Traits class + named parameters for MinMeanCycle (#179)

- Add a Traits class defining LargeValue, Tolerance, Path types.
LargeValue is used for internal computations, it is 'long long'
if the length type is integer, otherwise it is 'double'.
- Add named template parameters for LargeValue and Path types.
- Improve numerical stability: remove divisions from the internal
computations. If the arc lengths are integers, then all used
values are integers (except for the cycleMean() query function,
of course).

 @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