decompose: merge decompose_strength() and decompose_scc()
These two functions were doing almost identical work, the only difference was the way to select the SCC to keep. Now we have a more uniform way to do that. Closes #172. * bin/autfilt.cc: Offer a unique --decompose-scc option, but keep --decompose-strength as an alias for backward compatibility. * spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: Rename decompose_strength as decompose_scc, and handle a way to list all SCC numers in the string specifier. This gets rid of the nearly identical * tests/core/scc.test, tests/core/strength.test, tests/python/decompose.ipynb, tests/python/decompose_scc.py: Adjust test cases. * NEWS: Adjust.