estimate growth parameters consistently

[kellijohnson-NOAA]

Problem

There are many ways to estimate growth parameters and it is not consistent between nwfscSurvey and PacFIN.Utilities. Furthermore, parameter estimates are different depending on the parameterization.

I had originally started a new branch to do this, but it moved from the icebox to a broken down unplugged freezer. I plan on

• deleting the branch vbgrowth as part of Clean-up existing branches  #50 and
• documenting needs here for a fresh start.

Proposed solution

Implement many ways of estimating growth. Talk to @shcaba and @melissamonk-NOAA because they both do this independently of the packages. Maybe even @ChristineStawitz-NOAA.

• nwfscSurvey::PlotVarLengthAtAge.fn() parameterized in terms of L0 and not t0, @melissamonk-NOAA has ideas
• not sure about the lack of log(pars) in nwfscSurvey version
• bells and whistles can be added to nwfscSurvey::PlotVarLengthAtAge.fn(); clear labels for axes; alternative text

[chantelwetzel-noaa]

An approach for estimating growth in terms of L0 has been created within the new est_growth function. This function works similar to the approach in the {PacFIN.Utilities} package where a predicted length along with a ranged of potential low and high values based on a pre-specified number of standard deviations. The est_growth function calls the same function as the {PacFIN.Utilities} package which is called fit_vbgrowth and is housed in the {nwfscSurvey} package.