Add basic SANS workflow

[SimonHeybrock]

After #159, replace the placeholder with an actual workflow.

• Keep it minimal for now, compute IofQ[SampleRun] without background run subtraction.
• Do not use other inputs files, if it can be avoided. For example mask files and direct-beam files are not needed. This will be dealt with in follow-up.
• Set reasonable defaults for all input parameters. Workflow configuration has not been implemented yet and is not part of this task.
• The workflow should use the JSONGroup that is passed to the workflow as the input sample "file".
• Also return some other results, such as monitor histograms. This will be useful for testing handling of multiple outputs by the visualization module.

Use this:

• https://scipp.github.io/esssans/user-guide/loki/loki-direct-beam.html#Define-reduction-parameters
• https://scipp.github.io/esssans/user-guide/loki/loki-direct-beam.html#Create-pipeline-using-Sciline
• What should we do about the empty beam run? What if it does not exist yet?

[YooSunYoung]

I'll clear the milestone from this issue since we implemented #186 instead for the current milestone

[SimonHeybrock]

Picking this up now, in a modified form. Plan is as follows:

• We have already successfully split event-load from the rest of the load.
• Run workflow to compute all static intermediate results.
• In a loop, load chunk of events and run the workflow until right before normalization. Here we need to accumulate, i.e., add to previous values.
• Compute I(Q) based on accumulated values.

This seems to be possible with some minor and very sensible changes to the existing SANS workflow.