Rework and fix the implementation of MinMeanCycle (#179)

- Fix the handling of the cycle means.
- Many implementation improvements:
- More efficient data storage for the strongly connected
components.
- Better handling of BFS queues.
- Merge consecutive BFS searches (perform two BFS searches
instead of three).

This version is about two times faster on average and an order of
magnitude faster if there are a lot of strongly connected components.

 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)