sat_minimize: improve logs and document Python bindings

* spot/priv/satcommon.cc, spot/priv/satcommon.hh: Make it possible to
set the log file without setting the environment variable.  Adjust
print_log to take the input state and print it as a new column.
* spot/twaalgos/dtbasat.cc, spot/twaalgos/dtwasat.cc: Adjust all
calls to print_log.  Fix log output for incremental approaches.
Prefer purge_unreachable_states() over stats_reachable().  Do
not call scc_filter() on colored automata.
* spot/twaalgos/dtwasat.hh: Document the new "log" option.
* NEWS: Mention the changes.
* tests/python/satmin.ipynb: New file.
* tests/Makefile.am: Add it.
* doc/org/satmin.org, doc/org/tut.org: Link to it.
* doc/org/satmin.org, bin/man/spot-x.x: Adjust description
of CSV files.
* bench/dtgbasat/gen.py, bench/dtgbasat/tabl.pl,
bench/dtgbasat/tabl1.pl, bench/dtgbasat/tabl2.pl,
bench/dtgbasat/tabl3.pl, bench/dtgbasat/tabl4.pl: Adjust for
the new column.
* spot/misc/satsolver.cc, spot/misc/satsolver.hh (stats): Make it
const.
* python/spot/__init__.py (sat_minimize): Add display_log and
return_log options.
* tests/python/ipnbdoctest.py: Adjust to not compare SAT-minimization
logs as they contain timings.

bench/dtgbasat/gen.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-# Copyright (C) 2016, 2017 Laboratoire de Recherche et Développement de
+# Copyright (C) 2016-2018 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -272,7 +272,7 @@ def get_last_successful(n, category, pattern):
                    + '.satlog', 'r')
         log_csv = csv.reader(log)
         for line in log_csv:
-            min_val = line[1]
+            min_val = line[2]
         return '$\\le$' + min_val
     except Exception:
         return ''