SGRefObjects not allowed to share RefCount object any more.
Fixed unit test; moved threaded stress_test fo RefCount_unittest.
Parallel and SGIO should not copy ref-count values with copy-constructor;
rather maintain ref-counts on their own.
Removing obsolete test helper stress_test() - which is now in RefCount_unittest.
Merge pull request #2218 from tklein23/fix_refcounts_in_sgrefobject

SGRefObjects not allowed to share RefCount objects (any more)
C4.5 algorithm added
Merge pull request #2220 from mazumdarparijat/C45

C4.5 algorithm added
added opencv as the optional dependency and a basic test in tests/integration/opencv.
added flags for opencv integration in CMakeLists.txt.
shortened the flag name for OpenCV integration
Merge branch 'develop' of https://github.com/kislayabhi/shogun into develop