Traits class + named parameters for MinMeanCycle (#179)

- Add a Traits class defining LargeValue, Tolerance, Path types.
LargeValue is used for internal computations, it is 'long long'
if the length type is integer, otherwise it is 'double'.
- Add named template parameters for LargeValue and Path types.
- Improve numerical stability: remove divisions from the internal
computations. If the arc lengths are integers, then all used
values are integers (except for the cycleMean() query function,
of course).

 SET(LEMON_INCLUDE_DIR "@CMAKE_INSTALL_PREFIX@/include" CACHE PATH "LEMON include directory")

 SET(LEMON_INCLUDE_DIRS "${LEMON_INCLUDE_DIR}")

 IF(UNIX)

   SET(LEMON_LIB_NAME "libemon.a")

 ELSEIF(WIN32)

   SET(LEMON_LIB_NAME "lemon.lib")

 ENDIF(UNIX)

 SET(LEMON_LIBRARY "@CMAKE_INSTALL_PREFIX@/lib/${LEMON_LIB_NAME}" CACHE FILEPATH "LEMON library")

 SET(LEMON_LIBRARIES "${LEMON_LIBRARY}")

 MARK_AS_ADVANCED(LEMON_LIBRARY LEMON_INCLUDE_DIR)