Add modbus_read service

Hi,

It would be great if the modbus platform could be extended with read or update services to force an update of the sensors defined. Now I have to cyclicly update my sensors avery 10 seconds to get reasonable response times although hardly anything happens during a day.

EDIT: I figured out how to do this, leaving it here for others to find.

Home Assistant has a service homeassistant.update_entity which you can call on any entity to force an update. This also works for Modbus. So right now I have defined a very slow update cycle and only update when needed from an automation (or AppDaemon app).

Thomas