resolves #77: fixes bug where an nd2 with a single cycle or z-level wouldn't work

jim.py

@@ -0,0 +1,4 @@
+import nd2reader
+nd2 = nd2reader.Nd2("/var/nd2s/hawkjo.nd2")
+
+print(nd2)

nd2reader/parser.py

@@ -234,8 +234,10 @@ class Nd2Parser(object):
         except AttributeError:
             return [0]
         except TypeError:
-            count = int(re.match(pattern, self._dimensions.decode("utf8")).group(1))
-            return list(range(count))
+            match = re.match(pattern, self._dimensions.decode("utf8"))
+            if not match:
+                return [0]
+            return list(range(int(match.group(1))))
         else:
             return list(range(count))