SimpleMap and SimpleWriteMap
- Trivial adaptors, but they are useful in some case

Some combined maps will be reference map if the first
template parameter map is reference map or not. If I want
to give a refernce map as first map but there is a non
reference map parameter then I should wrap my first map
to a regular read-write map.

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