Pull Request for Issue842: SR570 sensitivity PV: sens_put, sens_num and sens_unit are out of synch

@dretrex @dretrex @beaured0 

SR570 `sens_put`, `sens_num`, and `sens_unit` are out of synch when we set sens_put.
Solution:
  -- add the function to enable/disable PV record processing. when `.DISA` and `.DISV` are equal, PV won't populate its values to other PVs. 
  -- when we set `sens_put`, we will manually update the value of `sens_num` and that of `sens_unit`: (disable processing record, set value, reenable processing record).