[Feature]: add option to input matrix of M by year & age

[iantaylor-NOAA]

Describe the solution you would like.

As discussed during ongoing ICES course on SS3, expanding the M option 3 to allow a matrix of M by age and year would make life easier for those with SCAA-style models that include a matrix of M.

Describe alternatives you have considered

Status-quo option requires annual blocks on M-at-age parameters.

As noted by Max (@akatan999), for EWAA models, it might be nice to input an M matrix in the wtatage.ss file. However, this would prevent it's use in a model with parametric growth and maybe require more complex coding to implement.

Statistical validity, if applicable

No response

Describe if this is needed for a management application

No response

Additional context

No response