RMSE for recruitment predictions

[MOshima-PIFSC]

Suggestion by Max and contribution of code from Valerio to calculate RMSE for recruitment predictions from the retrospective output for different options of how recruitment is predicted. (code sent in email thread "Two ideas/suggestions for improving ss3diags" from Max).

[MOshima-PIFSC]

@efletcherPIFSC I looked at the code last week and started working on generalizing the function to work with the package. I still need to do more tests but should be able to complete for the next release.

[efletcherPIFSC]

@MOshima-PIFSC I can not recall if this was (a) implemented or (b) put on hold.

[MOshima-PIFSC]

@efletcherPIFSC this was put on hold