Fix Pcbnew Python: LoadBoard does not rebuild connectivity database, and later, SaveBoard does not save net class memberships

Fixes: lp:1711268
https://bugs.launchpad.net/kicad/+bug/1711268

pcbnew/python/examples/hidePcbValuesShowReferences.py

@@ -6,9 +6,9 @@ filename=sys.argv[1]
 
 pcb = LoadBoard(filename)
 
-for module in pcb.GetModules():    
+for module in pcb.GetModules():
     print "* Module: %s"%module.GetReference()
     module.Value().SetVisible(False)      # set Value as Hidden
     module.Reference().SetVisible(True)   # set Reference as Visible
-    
-pcb.Save("mod_"+filename)	
+
+SaveBoard("mod_"+filename, pcb)