Clarify semantics around `NXroot/partial`

[lukaspie]

Currently, the docs say:

A list of concepts in an application definition this file describes.
This is for partially filling an application definition.
If this attribute is not present the application definition is assumed
to be valid, if not only the specified concepts/paths are assumed to be valid.   

We should clarify how to write the list, i.e. an array of strings. Maybe some more documentation on how to use this is needed as well.

cc @mkuehbach

[mkuehbach]

@lukaspie NXroot on the NIAC side nexusformat@8e52ff5 does not have such concept, but on the fairmat side

nexus_definitions/base_classes/NXroot.nxdl.xml

Line 100 in 1e8c433

<attribute name="partial">

Is it currently in use? The docstring formulation is unnecessary tricky to read, have we proposed it towards the NIAC, if so where?

So more context to this issue would be useful to support an implementation.

[lukaspie]

This came from the technology partners, with the idea that they could write partial files, i.e. those where only a subset of concepts fulfill the application definition. I still think it is an incredible helpful idea, but when I proposed it in nexusformat#1418 (comment), it was quickly shut down, both because there was too much on the plate and because NIAC did not want to enter another rabbit whole.

I intentionally left it in our branch, to give us the chance to come up with a better proposal at some point.

We should anyway think how we want the fairmat branch to look like after the release. Shall it be an exact copy of the main branch from NIAC? Or do we rather keep it as what it was, the development branch for our proposal. My suggestion is that we keep the main branch on our side in sync with nexusformat/main. We can then also use this to make a stable version of pynxtools (aka a version 1.0) and then subsequent development versions can use fairmat again. In any case, these will be questions that come up.

[tomio13]

Just a thought: we may help the technology partners by doing our best to keep the number of required fields minimal. Because only those are the must, and the result is a partial file in some sense.