[CN] Rename the extract statement to focus

[cp526]

as suggested by Benjamin ( rems-project/cn-tutorial#51 ). The current name "extract" does not reflect the semantics of the CN statement very well.

[dc-mak]

Also related: #498, #471, #361, #357, #320, #311, #256

[jprider63]

It sounds like this involves renaming "extract" to "focus" and adding a deprecation warning on "extract". Does that seem accurate @cp526? If so, maybe this is a good next issue for me to work on.

[bcpierce00]

In the spirit of getting as much "naming tidying" done as possible while some of our attention is focused there, I'd say yes...

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[jprider63]

Sounds good. If @cp526 is on board, I can look at that after adding benchmarking to CI.

[cp526]

Sounds good to me!