sccfilter: implement a new version of tgba_digraph
The new version currently supports removal of useless state as well as removal of acceptance sets from non-accepting SCCs (the two versions). It does not yet support simplifation of acceptance sets and removal of suspendable formulae. However the design, using filters that are composed before being applied, should make it easier to implement. * src/tgbaalgos/sccfilter.cc, src/tgbaalgos/sccfilter.hh: Implement the new scc_filter and supporting classes. * src/tgbaalgos/simulation.cc, src/tgbaalgos/simulation.hh: Use it. The simulation now always return a tgba_digraph. * src/tgbatest/sim.test: Adjust.