location and format of key in properties
In order to identify datastreams in our STA endpoint w/o having to query O2A, we need to have
the code of Thing|Sensor|Datastream in our STA properties.
This code is a combination of the (hopefully) unique short names of parent-items, sub-items and paramaters of O2A items. This shortname is supplied by the O2A API.
The O2A STA endpoint stores the code in the properties.code key of Sensor and Datastream, but for Things, it is located in the properties.metadata.code key.
I would suggest we agree to always store it in the properties.code, allowing for an additional properties.metadata.code field for compability.
The NRT format specs allow adding a unit after the code. I would suggest that we do not store the unit in the datastream.properties.code, i.e. that we completely ignore the unit when looking for a Datastream identified by the NRT column header.