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.

prefix=@prefix@

exec_prefix=@exec_prefix@

libdir=@libdir@

includedir=@includedir@

Name: @PACKAGE_NAME@

Description: Library of Efficient Models and Optimization in Networks

Version: @PACKAGE_VERSION@

Libs: -L${libdir} -lemon

Cflags: -I${includedir}