191 spawner recruitment with a b formulation

[Rick-Methot-NOAA]

Concisely (20 words or less) describe the issue

create new spawner-recruitment with a,b formulation

Please Link issue(s)

#191
resolves #191

What tests have been done? Upload any model input files created for testing in a zip file, if possible.

produces exact match to SRR=3 when tested without time-varying biology.

What tests/review still need to be done? Who can do it, and by when is it needed (ideally)?

Need to test with time-varying biology then add more output to show consequences of implied steepness changing over time.

Has any new code been documented?

• I have documented any new code added (or no new code was added)

is there an input change for users to Stock Synthesis?

• Yes, there was an input change

If so, please provide an example of the new inputs needed.

[New example stock synthesis input goes here]

Check which is true. This PR requires:

• no further changes to r4ss
• no further changes to the manual
• no further changes to SSI (the SS3 GUI)
• no further changes to the stock synthesis change log (new features, bug reports)

Describe any changes in r4ss/SS3 manual/SSI that are needed (if not checked):

If changes are needed in the change log, please fill in the table here:

Action	Topics	Type
[fix, new, or revise]	[e.g., biology. Use issue label options.]	[input, output, and/or calc, or ALL]

Additional information (optional):

[Rick-Methot-NOAA]

create example with time-vary growth (Linf increase by 5 cm). See impact on SPB/R in expanded output to SPAWN_RECR table; but very little impact on estimated recruitments. Implied steepness move from 0.6 to 0.63 with the change in SPB/R.
Note that r4ss will be broken for now. Need to cleanup the SPAWN_RECR table first.
@timjmiller/wham
compare_SRR.zip