Projection, Configuration and Start Document Metadata

[dylanmcreynolds]

The initial version of the projector contains a misunderstanding of the event-model documents structure.

I took the techniques PR from @tacaswell and implemnted "configuration" types as metadata from the run start document. I believe now that this was intended to be fields from the configuration section of the even descriptor document, which contains fields stored once per scan per stream.

I believe that projections should support both. So, I propose to change/enhance this soon.

[dylanmcreynolds]

I recently added a value field to to projections to support the static type. This type will let you defined a field and value in the projection, which will be added the resulting Dataset, directly. This was added to the event-model run_start schema (see bluesky/event-model#194)

Question though: when projecting this field into an xarray.Dataset, is there a default location that it should go into? Should the schema require that a location also be added so that the projector can treat it like one of:

• metadata
• event configuration
• event field

Or would it just go in the metadata? I'm sure I know the answer, but wanted to check with @ronpandolfi.

Not particularly germane to this PR but I'd like to sneak it in if I can.