Un00 units support

[rerpha]

Issue

#3

Description of work

Adds support for streaming units, in the form of the un00 schema, on PV updates.

logic here is:

• on initial message with blank units, sends blank update.
• on initial message with no EGU field at all (ie an mbbi), sends no update.
• on updated units, sends update.

To review this:

• (optionally) have a local redpanda instance using this
• clone and run saluki - how to here
• pull this branch into ISIS\forwarder\master
• do an editable pip install of the forwarder, but then git clone add un00 - engineering units ess-dmsc/python-streaming-data-types#107 and pip install it locally
• run a server
• use saluki listen livedata:31092/YOURMACHINE_sampleEnv and change the units (.EGU field) of a PV (ideally one you've made a block for as it'll get picked up by BSKAFKA) and see that a un00 gets published with the new units.

Checklist

• Pre-commit hooks have been run (see https://github.com/ess-dmsc/forwarder#developer-information)
• Changes have been documented in changes.md