Add J/kg and grains/m³ units

[obones]

In the context of writing the OpenMeteo addon, I'm creating channels for various measurements, two of which need units that are not yet available in openHAB:

Measurement	Unit
CAPE	J/kg
Pollen concentration	grains/m³

I have tried adding them in my addon but they can't be used, as indicated in the forum

Your Environment

• Version used: 4.2.0 snapshot
• Environment name and version : Java 17
• Operating System and version : Windows 10-11

[openhab-bot]

This issue has been mentioned on openHAB Community. There might be relevant details there:

https://community.openhab.org/t/declare-new-units-in-an-addon/156993/5