Allow use of SSB_unfished as the Bratio denominator

[k-doering-NOAA]

Thanks @akatan999 for this suggestion!

In the long term, it would be very useful to add an option where Bratio can [use either] SSB_unfished or to SSB_Virgin [as the denominator] at the choice of the user directly when setting the starter file.

I think this would mean creating a new depletion basis option in the starter file that allowed the user to use SSB_unfished as the denominator.

[Rick-Methot-NOAA]

This will be tricky to implement. SSB_unfished is not calculated until the benchmark routine and depletion is calculated during the time series.

[e-perl-NOAA]

@Rick-Methot-NOAA should we close this issue since it's marked as "wont-fix"?

[Rick-Methot-NOAA]

#442 is related issue. Closing here and will mention there.