acc: make &= and |= symmetrical

Operator &= used to always move Fin to the front, it does not anymore.
The only thing it does now is to merge Inf(x)&Inf(y) as Inf({x,y}).
Operator |= is now symmetrical and merges Fin()s.

Fixes #253.

* spot/twa/acc.cc, spot/twa/acc.hh: Simplify &= and make |= symmetrical.
* spot/twaalgos/cleanacc.cc: Fix conjunction order.
* tests/core/acc.test, tests/core/acc2.test, tests/core/parseaut.test,
tests/core/readsave.test, tests/core/satmin2.test,
tests/core/sccdot.test, tests/python/acc_cond.ipynb,
tests/python/accparse.ipynb, tests/python/automata.ipynb,
tests/python/product.ipynb, tests/python/randaut.ipynb: Adjust test
cases.

NEWS

@@ -156,6 +156,12 @@ New in spot 2.3.5.dev (not yet released)
   - If the SPOT_BDD_TRACE envorinment variable is set, statistics
     about BDD garbage collection and table resizing are shown.
 
+  - The & and | operators for acceptannce conditions have been changed
+    slightly to be more symmetrical.  In older version, operator &
+    would move Fin() terms to the front, but that is not the case
+    anymore.  Also operator & was already grouping all Inf() terms
+    (for efficiency reasons), in this version operator | is
+    symmetrically grouping all Fin() terms.
 
   Python: