postproc: fix default for acd and interaction with colored
- spot/twaalgos/postproc.hh (postprocess::acd_): Default to true.
- spot/twaalgos/postproc.cc (postprocess::run): When acd is used to color an automaton, do not run scc_filter to remove color from transiant edges.
- tests/python/acd.py: New file.
- tests/Makefile.am: Add it.