For #184.

* spot/graph/graph.hh, spot/kripke/kripkegraph.hh,
spot/misc/bitvect.hh, spot/misc/common.hh, spot/misc/fixpool.hh,
spot/misc/mspool.hh, spot/misc/timer.hh, spot/tl/formula.hh,
spot/twa/acc.hh, spot/twa/taatgba.hh, spot/twa/twa.hh,
spot/twa/twagraph.hh, spot/twaalgos/emptiness_stats.hh,
spot/twaalgos/mask.hh, spot/twaalgos/ndfs_result.hxx,
spot/twaalgos/sccinfo.hh, spot/twaalgos/translate.hh: Replace
assert() by SPOT_ASSERT(), or an exception, or nothing, depending
on the case.
* tests/sanity/style.test: Flag all asserts in headers.
* HACKING: Discuss assertions.