Swarming implies that a single instance of the kripke
structure (or product) will be explored by diffrent threads
with their own exploration order. Most of the modification
aims to have a thread safe kripke structure.

* spot/kripke/kripke.hh, spot/ltsmin/ltsmin.cc,
spot/ltsmin/ltsmin.hh, spot/mc/ec.hh,
spot/mc/intersect.hh, spot/mc/reachability.hh,
spot/misc/hash.hh, spot/twacube/twacube.hh,
tests/core/twacube.test, tests/ltsmin/modelcheck.cc: here.