specialize scc_filter for inherently_weak automata

Part of issue #351.

* spot/twaalgos/sccfilter.cc, spot/twaalgos/sccfilter.hh: Specialize
for inherently-weak automata.
* spot/twaalgos/postproc.cc: Simplify.
* tests/core/dca2.test, tests/core/parity2.test,
tests/core/prodor.test, tests/core/randomize.test,
tests/python/automata.ipynb, tests/python/highlighting.ipynb,
tests/python/product.ipynb, tests/python/remfin.py,
tests/python/stutter-inv.ipynb: Adjust.
* NEWS: Mention it.

tests/core/parity2.test

@@ -236,7 +236,7 @@ AP: 1 "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
 properties: trans-labels explicit-labels state-acc stutter-invariant
-properties: inherently-weak
+properties: weak
 --BODY--
 State: 0
 [t] 0
@@ -270,7 +270,7 @@ AP: 1 "a"
 acc-name: Rabin 1
 Acceptance: 2 Fin(0) & Inf(1)
 properties: trans-labels explicit-labels state-acc stutter-invariant
-properties: inherently-weak
+properties: weak
 --BODY--
 State: 0
 [t] 0
@@ -304,7 +304,7 @@ AP: 1 "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
 properties: trans-labels explicit-labels state-acc stutter-invariant
-properties: inherently-weak
+properties: weak
 --BODY--
 State: 0
 [t] 0
@@ -338,7 +338,7 @@ AP: 1 "a"
 acc-name: Streett 1
 Acceptance: 2 Fin(0) | Inf(1)
 properties: trans-labels explicit-labels state-acc colored
-properties: stutter-invariant inherently-weak
+properties: stutter-invariant weak
 --BODY--
 State: 0 {0}
 [t] 0
@@ -372,7 +372,7 @@ AP: 1 "a"
 acc-name: parity min odd 3
 Acceptance: 3 Fin(0) & (Inf(1) | Fin(2))
 properties: trans-labels explicit-labels state-acc colored
-properties: stutter-invariant inherently-weak
+properties: stutter-invariant weak
 --BODY--
 State: 0 {2}
 [t] 0
@@ -406,7 +406,7 @@ AP: 1 "a"
 acc-name: parity max even 3
 Acceptance: 3 Inf(2) | (Fin(1) & Inf(0))
 properties: trans-labels explicit-labels state-acc colored
-properties: stutter-invariant inherently-weak
+properties: stutter-invariant weak
 --BODY--
 State: 0 {1}
 [t] 0