Less map copying in NetworkSimplex (#234)

- The graph is copied in the constructor instead of the init() function.
It must not be modified after the class is constructed.
- The maps are copied once (instead of twice).
- Remove FlowMap, PotentialMap typedefs and flowMap(), pontentialMap()
setter functions.
- flowMap() and potentialMap() query functions copy the values into the
given map (reference) instead of returning a const reference to a
previously constructed map.

@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