[Feature]: Add age range option to 2DAR selectivity

Describe the solution you would like.

Existing 2DAR options include ability to extend last dev to all remaining years, similarly for early years.

Suggestion here is to add similar capability for ages (or lengths) such that the dev for the youngest age in 2DAR could extend to all younger ages; similarly for older ages (or lengths).

This extended set of options could be invoked by invoking 2DAR with "2", rather than "1".

Describe alternatives you have considered

none

Statistical validity, if applicable

seems logical and useful to me

Describe if this is needed for a management application

probably could be used by the hake model

Additional context

No response