StringTree.value(): Add optional path argument

As opposed to the C++ variant, StringTree.value() doesn't take a path
argument, but only returns the value of the node it's called on.
Change this. Returns None if the path is not found.

Signed-off-by: Jan Lindemann <jan@janware.com>

tools/python/jwutils/stree/StringTree.py

@@ -115,7 +115,7 @@ class StringTree: # export
         r = self.get(path)
         if r is None:
             raise KeyError(path)
-        return r.value()
+        return r.value() # type: ignore
 
     def __setitem__(self, key, value):
         return self.__set(key, value)
@@ -170,7 +170,12 @@ class StringTree: # export
         relpath = '.'.join(components[1:])
         return self.children[name].get(relpath)
 
-    def value(self) -> str:
+    def value(self, path = None) -> Optional[str]:
+        if path:
+            child = self.get(path)
+            if child is None:
+                return None
+            return child.value()
         if len(self.children) == 0:
             raise Exception('tried to get value from leave "{}"'.format(self.content))
         return self.children[next(reversed(self.children))].content # type: ignore