Document and greatly improve TSP algorithms (#386)

- Add LEMON headers.
- Add Doxygen doc for all classes and their members.
- Clarify and unify the public API of the algorithms.
- Various small improvements in the implementations to make
them clearer and faster.
- Avoid using adaptors in ChristofidesTsp.

 @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