Swapping logic flexibility

[forero]

In the current implementation fibers can swap targets across different layers to increase the numbers of high priority targets.

We need a way to implement swapping logic to instruct the code to:

• swap only within a layer
• swap only within a single tile
• never swap

[sbailey]

Another variation: allow swapping of targets of the same priority, but not swapping across priorities.

Example: Currently the code could bump an LRG from layer 0 to an unused fiber in layer 2 in order to observe an ELG in layer 0 that otherwise would have been lost. This increases the total number of targets observed, but has the "bird in the hand vs. two in the bush" risk that a higher priority target (LRG) was moved to a later tile. We may not want to allow that, even if we continue to allow swapping ELGs between layers in order to get more ELGs.

The current mood in the clustering WG is to not allow swapping at all (or maybe within a tile but not across tiles) unless it is demonstrated that it really is a large gain in number of targets. We need to quantify the gain with the current code to evaluate that.

[tskisner]

The current code (Spring 2020) does not do "swapping". High priority targets are assigned preferentially to early tiles. I think we can close this, but will let @sbailey make the final call.