text: Title: dashboard error
  Body: ## Mycodo Issue Report:
    - Specific Mycodo Version: 6.08
   
  #### Problem Description
  Please list: 
  
    - when i modify the dashboard and add in a graph two value: temp setpoint, temp from ds18b20
    
   
  ### Errors
    - List any errors you encountered.
    - Traceback (most recent call last):
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask/app.py", line 1982, in wsgi_app
      response = self.full_dispatch_request()
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask/app.py", line 1614, in full_dispatch_request
      rv = self.handle_user_exception(e)
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask/app.py", line 1517, in handle_user_exception
      reraise(exc_type, exc_value, tb)
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask/_compat.py", line 33, in reraise
      raise value
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask/app.py", line 1612, in full_dispatch_request
      rv = self.dispatch_request()
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask/app.py", line 1598, in dispatch_request
      return self.view_functions[rule.endpoint](**req.view_args)
    File "/home/pi/Mycodo/env/lib/python3.5/site-packages/flask_login/utils.py", line 261, in decorated_view
      return func(*args, **kwargs)
    File "/home/pi/Mycodo/mycodo/mycodo_flask/routes_page.py", line 400, in page_dashboard
      y_axes = utils_dashboard.graph_y_axes(dict_measurements)
    File "/home/pi/Mycodo/mycodo/mycodo_flask/utils/utils_dashboard.py", line 469, in graph_y_axes
      input_dev)
    File "/home/pi/Mycodo/mycodo/mycodo_flask/utils/utils_dashboard.py", line 538, in check_func
      elif each_device.cmd_measurement and each_device.cmd_measurement != '':
  AttributeError: 'PID' object has no attribute 'cmd_measurement'
  
  
  ### Steps to Reproduce the issue:
  How can this issue be reproduced?
   1.  create data for ds18b20
   2.  create function Pid
   3.  add to graph : setpoint pid, ds18b20 temp
   
  