make StringType default maximum size = 255 Bytes

Change-Id: I0c26fc24ad99ff4185ff5a0b9b8ada9a10817b8b
Reviewed-on: https://forge.frm2.tum.de/review/16269
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

secop/datatypes.py

@@ -260,7 +260,7 @@ class StringType(DataType):
     minsize = None
     maxsize = None
 
-    def __init__(self, maxsize=None, minsize=0):
+    def __init__(self, maxsize=255, minsize=0):
         if maxsize is None:
             raise ValueError('StringType needs a maximum bytes count!')
         minsize, maxsize = min(minsize, maxsize), max(minsize, maxsize)