Add ability to run a joint profile

[chantelwetzel-noaa]

I think it would be useful to add a function to create a joint profile across two parameters. I have some existing code that could be modified that would facilitate the development of this approach. Currently, I have plotted joint profile space via contour plots (see below). I wanted to create an issue on this topic so users could weigh in what type of figures and tables would be most useful. I think @shcaba has already done some work on this front and would like to make sure that we incorporate approaches that other have been found useful.

[shcaba]

Multi-parameter likelihoods can be run in the SS-DL (https://github.com/shcaba/SS-DL-tool), but I'd be happy to work to isolate that code for this package.
It is different from the grid search approach being shown in the example above. Maybe we have both options available.

[k-doering-NOAA]

Great idea! I wonder if this belongs in r4ss rather than nwfsc-assess? It sounds general to SS models?

Of course, it could always be moved at a different time, if it seems best to start working on it in nwfsc-assess instead.

[iantaylor-NOAA]

Both SS_DL and nwfscDiag use r4ss::SS_profile(), but wrap it up in a more user-friendly way. The bivariate profile option in r4ss is discussed in r4ss/r4ss#224. I'm on the fence about the pros and cons of incorporating something like those wrappers into r4ss, but it definitely would make sense to add support for some kind of plot of bivariate profile results.