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.

tests/core/sccdot.test

@@ -74,7 +74,7 @@ run 0 autfilt --dot=as in.hoa > out.dot
 cat <<EOF >expected
 digraph G {
   rankdir=LR
-  label="Fin(2) & (Inf(0)&Inf(1))"
+  label="(Inf(0)&Inf(1)) & Fin(2)"
   labelloc="t"
   node [shape="circle"]
   I [label="", style=invis, width=0]
@@ -156,7 +156,7 @@ HOA: v1
 States: 8
 Start: 0
 AP: 2 "a" "b"
-Acceptance: 3 Fin(2) & (Inf(0)&Inf(1))
+Acceptance: 3 (Inf(0)&Inf(1)) & Fin(2)
 properties: trans-labels explicit-labels trans-acc
 --BODY--
 State: 0