* python/spot/__init__.py: Add 'assume' option.
* spot/misc/satsolver.cc: Add function to handle assumptions.
* spot/misc/satsolver.hh: Declare assumption function.
* spot/twaalgos/dtbasat.cc: Implement dtba_sat_minimize_assume.
* spot/twaalgos/dtbasat.hh: Declare it.
* spot/twaalgos/dtwasat.cc: Implement dtwa_sat_minimize_assume and
handle options.
* spot/twaalgos/dtwasat.hh: Declare it.
* spot/twaalgos/postproc.cc: Handle options.
* spot/twaalgos/postproc.hh: Use param_ var for incr and assume.
* tests/core/satmin.test: Add tests for the new function.
* tests/core/satmin2.test: Add tests for the new function.
* tests/python/satmin.py: Add tests for the new function.