Support >= and <= constraints in NetworkSimplex (#219, #234)

By default the same inequality constraints are supported as by
Circulation (the GEQ form), but the LEQ form can also be selected
using the problemType() function.

The documentation of the min. cost flow module is reworked and
extended with important notes and explanations about the different
variants of the problem and about the dual solution and optimality
conditions.

 INCLUDE(FindPackageHandleStandardArgs)

 FIND_PROGRAM(GHOSTSCRIPT_EXECUTABLE

   NAMES gs gswin32c

   PATHS "$ENV{ProgramFiles}/gs"

   PATH_SUFFIXES gs8.61/bin gs8.62/bin gs8.63/bin gs8.64/bin gs8.65/bin

   DOC "Ghostscript: PostScript and PDF language interpreter and previewer."

 )

 FIND_PACKAGE_HANDLE_STANDARD_ARGS(Ghostscript DEFAULT_MSG GHOSTSCRIPT_EXECUTABLE)