decompose: merge decompose_strength() and decompose_scc()

These two functions were doing almost identical work, the only
difference was the way to select the SCC to keep.  Now we have a more
uniform way to do that.  Closes #172.

* bin/autfilt.cc: Offer a unique --decompose-scc option, but keep
--decompose-strength as an alias for backward compatibility.
* spot/twaalgos/strength.cc, spot/twaalgos/strength.hh: Rename
decompose_strength as decompose_scc, and handle a way to list
all SCC numers in the string specifier.  This gets rid
of the nearly identical
* tests/core/scc.test, tests/core/strength.test,
tests/python/decompose.ipynb, tests/python/decompose_scc.py: Adjust
test cases.
* NEWS: Adjust.

NEWS

@@ -21,6 +21,10 @@ New in spot 2.3.5.dev (not yet released)
   - autfilt learned --simplify-acceptance to simplify some acceptance
     conditions.  (See spot::simplify_acceptance() below.)
 
+  - autfilt --decompote-strength has been renamed to --decompose-scc
+    because it can now extract the subautomaton leading to an SCC
+    specified by number.  (The old name is still kept as an alias.)
+
   Library:
 
   - A new library, libspotgen, gathers all functions used to generate
@@ -41,6 +45,12 @@ New in spot 2.3.5.dev (not yet released)
 
   - spot::dtwa_complement now simply returns the result of dualize()
 
+  - spot::decompose_strength() was extended and renamed to
+    spot::decompose_scc() as it can now also extract a subautomaton
+    leading to a particular SCC.  A demonstration of this feature via
+    the Python bindings can be found at
+    https://spot.lrde.epita.fr/ipynb/decompose.html
+
   - A new named property for automata called "original-states" can be
     used to record the origin of a state before transformation.  It is
     currently defined by the degeneralization algorithms, and by
@@ -161,6 +171,9 @@ New in spot 2.3.5.dev (not yet released)
 
   Deprecation notice:
 
+  - spot::decompose_strength() is deprecated, it has been renamed
+    to spot::decompose_scc().
+
   - spot::dtwa_complement() is deprecated.  Prefer the more generic
     spot::dualize() instead.
 
@@ -330,11 +343,6 @@ New in spot 2.3.1 (2017-02-20)
     Kupferman & Rosenberg [MoChArt'10] are recognizable by
     deterministic Büchi automata with at least 2^2^n states.
 
-  - autfilt has a new transformation: --decompose-scc, which allows
-    decomposition of automata through their accepting SCCs.
-    A demonstration of this feature via the Python bindings
-    can be found at https://spot.lrde.epita.fr/ipynb/decompose.html
-
   Library:
 
   - spot::twa_run::as_twa() has an option to preserve state names.