(core) Basic structure of MQTT publishing
(core) Use scala-mqtt to publish to the broker.
(core) Genericise the MQTT client used.

Since we're only using the publish-side of the client, we can abstract
over the "stream" type.
(core) Update sample Main to reflect MQTT client
(core) The MQTT ClientModule is required ...

... in addition to an already connected Client.  Since the client is
dependently typed (on the module), it causes complications with passing
the client and the module into a class constructor.

The real problem here is a that the scala-mqtt API is not as good as it
should be.
(core) Basic measurement serialisation.

More just a placeholder for now.
(core) update sample Main
Merge branch 'feature-publish-to-mqtt'