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

 ./preflow_demo_leda < $1

 ./preflow_demo_boost < $1

 ./preflow_demo_jacint < $1

 ./preflow_demo_athos < $1

 ./edmonds_karp_demo < $1

 ./edmonds_karp_demo_alpar < $1

 ./edmonds_karp_demo_boost < $1