Clarify the operations of reduceArc and reduceExposure

Following the discussion during the Princeton Group meeting 2021-05-17: please clarify the inputs, outputs and operations performed in the reduceExposure and reduceArc tasks in the context of arc line processing.

Specifically:

• Specify where in the code the detectorMap is modified, and how (and is it overridden, stated in PIPE2D-844?)
• Specify where the arcLines output is generated, and the inputs and algorithm used to generate this product. And again, if these are generated by reduceExposure, are they overridden in reduceArc??

This information can be provided as comments in this ticket, for the time being. If the approach is agreed within the Princeton Group, the design document should be updated.