* python/spot/__init__.py: Add 'incr' boolean argument.
* spot/twaalgos/dtbasat.cc: Implement dtba_sat_minimize_incr(...).
* spot/twaalgos/dtbasat.hh: Declare it.
* spot/twaalgos/dtwasat.cc: Implement dtwa_sat_minimize_incr(...) and
deal with options.
* spot/twaalgos/dtwasat.hh: Declare it.
* spot/twaalgos/postproc.cc: Add option --sat-minimize=incr.
* spot/twaalgos/postproc.hh: Add incr parameter.
* tests/core/satmin.test: Add tests for incremental version.
Update expected result.
* tests/core/satmin2.test: Add tests for incremental version.
* tests/python/satmin.py: Add tests for incremental version.