point in using a map since the SCC are numbered in sequence.

* src/tgbaalgos/scc.hh (scc_map::relabel_component): Return the
number of the SCC instead of taking it as argument.
(scc_map::scc_num_): Delete this variable.  scc_map_.size() gives
the same information.
(scc_map::scc_map_type): Define using std::vector instead of
std::map.
* src/tgbaalgos/scc.cc: Adjust all uses.