Applied the changes which somehow vanished during my last merge. Thanks goes
to Marci for noticing this. In detail:
- added amsmath and amssymb latex packages for latex documentation
- src/demo is also scanned for doxygen input files

 c LEMON max flow problem

 p max 7 9

 n 1 s

 n 7 t

 a 1 2 3

 a 1 3 2

 a 1 4 1

 a 2 5 3

 a 3 5 2

 a 3 7 5

 a 3 6 3

 a 4 6 1

 a 5 7 2

 a 6 7 4