params: move unit into datatypes, support for constant property

Change-Id: Ida7c8b5fd7d508d41d3b44ea98600f27f7f57dbd
Reviewed-on: https://forge.frm2.tum.de/review/20243
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

secop/modules.py

@@ -204,7 +204,7 @@ class Module(object):
                 setattr(self, k, v)
             cfgdict.pop(k)
 
-        # Adopt units AFTER applying the cfgdict
+        # Modify units AFTER applying the cfgdict
         for k, v in self.accessibles.items():
             if not isinstance(v, Parameter):
                 continue