ENH: add schema for "projection" entry in start document

[tacaswell]

Provide a semantic mapping of the keys in the collected data to a known set of keys drawn from an externally owned vocabulary.

[danielballan]

This PR could use a description or a link to some meeting notes if there are any.

[danielballan]

On a Pilot call @dylanmcreynolds nudged us to move forward with this. I am personally happy with it but given the importance and the cost of any future changes I think we should have at least one more meeting to pick apart the structure and the names and consider alternatives.

[danielballan]

:+1: for @stuartcampbell's request to use this on a fully-worked example or three before we commit to it.

[danielballan]

@tacaswell on Slack:

move "techniques" out of start document and into [its] own document class

[danielballan]

I am in favor of proceeding on this but doing so in a separate experimental document type that can evolve quickly and make breaking changes as needed. I think @tacaswell suggested this in passing on Slack, as I tersely documented at the time, above.

[danielballan]

There was a call on this subject today.

Things there seemed to be strong consensus on:

• There will be a list of dicts, with each dict describing how to project the content of the documents on to some externally-defined schema. (In some situations there may be multiple ways to map the documents' contents onto the same schema; the same schema name might appear more than once. That is why why this is a list and not a dict keyed on schema name.) At top level, the dicts will describe the schema itself: a name ("NXtomo"), a version, an external URL to a schema definition, and finally a dict embodying the layout (e.g. Nexus definition).
• Within this layout, the "leaves" will be either pointers into a document (with the document name) or a literal value (with "literal" or something like that).
• This list of dicts will be included in the document stream, either within the Run Start document itself or in a temporary experimental document type (see below).

Things that might need more thought or discussion to build strong consensus:

• What to call the key: "techniques", "projections", or something else
• How the top-level keys (the name, version, url, etc.) are spelled and which are required
• How to structure and spell the pointers into documents
• Whether to add the notion of "experimental document type" to event-model, bluesky, and databroker so as to prototype this idea there, or just add this to the Run Start document from the get-go.

Things that need investigation:

• How to encode an XML attribute in JSON. @stuartcampbell reported that there are ~5 different standards for this. At some level we'd be happy to just pick one, but it's worth doing some due diligence regarding whether any of them have clear technical advantages or seem to have higher adoption than the others.

[prjemian]

First off, can someone please edit the top box here and describe clearly the intent of this PR, as previously requested? Without this focus stated, the discussion is not focused.

[dylanmcreynolds]

I have heard taht NeXus has deprecated XML backends. This leads to the question of how much XML you plan to support transforming to? Namespacing is great, but there seems to be a huge debate on how or if to represent that in JSON. It seems like you could punt on some complexity if you don't intend to output XML.

[prjemian]

That's right. HDF5 is now the only supported on-disk format for NeXus data files. The decision to drop the XML backend was between 2012 and 2014-08. (Can't find the specific decision in the notes yet.)