Make bitcell width/height not static. Update modules to use it for pbitcell.

compiler/modules/write_driver_array.py

@@ -58,11 +58,12 @@ class write_driver_array(design.design):
         self.mod_write_driver = getattr(c, OPTS.write_driver)
         self.driver = self.mod_write_driver("write_driver")
         self.add_mod(self.driver)
-        
+
+        # This is just used for measurements,
+        # so don't add the module
         c = reload(__import__(OPTS.bitcell))
         self.mod_bitcell = getattr(c, OPTS.bitcell)
         self.bitcell = self.mod_bitcell()
-        self.add_mod(self.bitcell)
 
     def create_write_array(self):
         self.driver_insts = {}