Rename Flow to Value in the flow algorithms (#266)

We agreed that using Flow for the value type is misleading, since
a flow should be rather a function on the arcs, not a single value.

This patch reverts the changes of [dacc2cee2b4c] for Preflow and
Circulation.

 #cmakedefine HAVE_LONG_LONG 1

 #cmakedefine HAVE_LP 1

 #cmakedefine HAVE_MIP 1

 #cmakedefine HAVE_GLPK 1

 #cmakedefine HAVE_CPLEX 1

 #cmakedefine HAVE_CLP 1

 #cmakedefine HAVE_CBC 1