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).

 INCLUDE_DIRECTORIES(

   ${PROJECT_SOURCE_DIR}

   ${PROJECT_BINARY_DIR}

 )

 LINK_DIRECTORIES(

   ${PROJECT_BINARY_DIR}/lemon

 )

 SET(DEMOS

   arg_parser_demo

   graph_to_eps_demo

   lgf_demo

 )

 FOREACH(DEMO_NAME ${DEMOS})

   ADD_EXECUTABLE(${DEMO_NAME} ${DEMO_NAME}.cc)

   TARGET_LINK_LIBRARIES(${DEMO_NAME} lemon)

 ENDFOREACH()