create new function switch between marginals and conditionals

[kellijohnson-NOAA]

Write a helper function that switches ages between marginals and conditionals.

• [x] assign someone to write this function
• [x] add this function to the package

[iantaylor-NOAA]

Now that we made a plan to do this, I think I can pull it off without too much work, so assigning myself with Friday deadline.

[iantaylor-NOAA]

Behind schedule on this, but adding task:

• [x] fix add_data() to make initial setting for survey marginal comps as ghost fleets, not part of likelihood as they are now by accident

[iantaylor-NOAA]

Function created a while ago, will be used for the last few sensitivities.