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.

 SET(PROJECT_NAME "@PACKAGE_NAME@")

 SET(PROJECT_VERSION "@PACKAGE_VERSION@" CACHE STRING "LEMON version string.")