[Feature]: Consider spline approach for M-at-age

[Rick-Methot-NOAA]

Describe the solution you would like.

add new option for M-at-age per the new approach in Multifan-CL paper: https://doi.org/10.1016/j.fishres.2023.106612

Describe alternatives you have considered

NA

Statistical validity, if applicable

paper: https://doi.org/10.1016/j.fishres.2023.106612

Describe if this is needed for a management application

No. This is exploratory.

Additional context

No response