This introduce some section for acceptance transformation, and add
more comments in a few places.

* spot/taalgos/emptinessta.hh, spot/twa/twa.hh, spot/twa/twagraph.hh,
spot/twaalgos/bfssteps.hh, spot/twaalgos/cleanacc.hh,
spot/twaalgos/degen.hh, spot/twaalgos/dualize.hh,
spot/twaalgos/parity.hh, spot/twaalgos/rabin2parity.hh,
spot/twaalgos/remfin.hh, spot/twaalgos/sccinfo.hh,
spot/twaalgos/sepsets.hh, spot/twaalgos/split.hh,
spot/twaalgos/totgba.hh: More doxygen.
* spot/twa/twagraph.cc: Typos in comments.