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
Removing old protobuf files before starting a new build.
Merge pull request #2223 from tklein23/remove_old_protobuf_files_before_build

Removing old protobuf files before starting a new build.
reduced accuracy in kernel row_col_wise_sum_block_with_diag unit-test to prevent buildbot failure
Merge pull request #2224 from lambday/develop

reduced accuracy in kernel row_col_wise_sum_block_with_diag unit-test to prevent buildbot failure
Fixed a bug in factor graph ipython notebook