Scyther.py: Added Caching to Scyther's Python interface.

Cached data is stored in:

Cache/XX/YYYYY.out (stdout)
Cache/XX/YYYYY.err (stderr)

Where XX^YYYYY is the sha256 hexdigest of the concatenation of the input spdl and
the arguments.

gui/Scyther/Misc.py

@@ -64,6 +64,40 @@ def sorted(li):
     return result
 
 
+# ensurePath: does what os.makedirs should do.
+def ensurePath(pt):
+    """
+    Make sure the path exists: if not, create the directories one by one
+
+    By example:
+
+    Call with "dog/cat/bone" ensures that afterwards, this subdirectory structure (dog/cat/bone) exists, with 'bone' a directory.
+    It ensures this by doing the procedure for "dog", then "dog/cat", etc...
+    """
+
+    ptn = os.path.normpath(pt)
+
+    # First we see what needs to exist overall
+    todo = [ptn]
+    tail = "x"
+    while len(tail) > 0:
+        (head,tail) = os.path.split(ptn)
+        if len(head) > 0:
+            todo.append(head)
+            ptn = head
+        else:
+            break
+
+    # Reverse list: path prefixes first
+    todo.reverse()
+
+    # Create bottom-up
+    for pt in todo:
+        if not os.path.isdir(pt):
+            # Note that os.path.exists(pt) may still hold. In this case the next command will cause an error.
+            os.mkdir(pt)
+
+
 # path
 def mypath(file):
     """ Construct a file path relative to the scyther-gui main directory