* spot/twaalgos/langmap.cc, spot/ltsmin/ltsmin.cc: Add asserts to hide
spurious "potential null pointer dereference" messages.
* spot/twaalgos/tau03opt.cc (color_ref): Initialize pc to nullptr
even when is_cyan is true so that valgrind does not report pc being
used uninitialized in is_white().
Fixes#203.
* spot/twaalgos/langmap.hh (highlight_languages): Simplify the
interface by only taking the automaton to color.
* spot/twaalgos/langmap.cc (highlight_languages): Only introduce
color for states that have a non-unique language.
* tests/core/highlightstate.test: Update and add more tests.
* tests/python/langmap.py: Keep the tests simple.
* bin/autfilt.cc: Adjust usage and help string.