suzerain: #3089 draft inputs/outputs API

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suzerain: #3089 stub .cpp file

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suzerain: #3089 move intf to treatment class

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suzerain: #3089 non-template constraint::treatment

Upside: quicker compiles and the possibility of using nicer
mean profile collection within the nonlinear operator RHS

Downside: introduces some virtual method overhead, but not
expected to be excessive.  Minimized as much as possible.

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