Implement add()
Implement add()

- this isn't strictly needed for replacing the dirq module
  as it's not called in ssm/ssm2.py
- however, I find it useful to be able to programmatically
  create messages in the outq when testing.
- and it might be useful for collector developers
- return a string (not a UUID object) because thats what the
  dirq module does

stfc/ssm,apel/ssm,apel/ssm,tofu-rocketry/ssm,tofu-rocketry/ssm,stfc/ssm