Make it clear this class is abstract

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/checks/format.py

@@ -204,7 +204,7 @@ class BaseFormatCheck(TargetCheck):
         return False
 
     def is_position_based(self, string):
-        return True
+        raise NotImplementedError()
 
     def check_single(self, source, target, unit, cache_slot):
         '''
@@ -251,6 +251,9 @@ class CFormatCheck(BaseFormatCheck):
     flag = 'c-format'
     regexp = C_PRINTF_MATCH
 
+    def is_position_based(self, string):
+        return True
+
 
 class PythonBraceFormatCheck(BaseFormatCheck):
     '''