Fixes#70.
We don't modify the behavior of mask_keep_states(), because it is
actually useful in some algorithm to remove states without renumbering
all the other states.
* src/bin/autfilt.cc: Call purge_dead_states().
* src/tgbatest/maskkeep.test: Adjust.
* src/tgbaalgos/mask.cc, src/tgbaalgos/mask.hh: Rename transform_mask
to accessible_mask. Add Copy version and use it in mask_keep_states().
* src/tgbatest/maskkeep.test: Update.
* src/bin/autfilt.cc: Add option --keep-states.
* src/tgbaalgos/mask.cc, src/tgbaalgos/mask.hh: Keep the selected states
and update the initial state.
* src/tgbatest/Makefile.am: Adjust.
* src/tgbatest/maskkeep.test: New file.
