Update recruitment, priors

[sebdalgarno]

Changes to API

• bb_predict_recruitment now uses DeCesare recruitment calculate decesare recruitment #29
• bb_predict_calf_cow_ratio now gives behaviour of old bb_predict_recruitment add bb_predict_calf_cow_ratio  #35

Other changes

• Recruitment model using Binomial on annual aggregated counts
• Update priors and vignettes for new recruitment model
• Survival model fixed year use intercept of 'good' year (i.e. high n and one mortality) resolve ML fixed year when intercept is difficult to estimate #33
• Loosened priors on survival intercept and bAnnual SD loosen priors on survival model  #38
• Fix bug for ML predictions and fixed year effect (missing first year) predict fixed year ML missing first year #32
• Update John orcid
• Add priors vignette add priors vignette #39
• Add example comparison plot to methods vignette