Update recruitment, priors

[sebdalgarno]

Changes to API

• bb_predict_recruitment now uses DeCesare recruitment #29
• bb_predict_calf_cow_ratio now gives behaviour of old bb_predict_recruitment #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) #33
• Loosened priors on survival intercept and bAnnual SD #38
• Fix bug for ML predictions and fixed year effect (missing first year) #32
• Update John orcid
• Add priors vignette #39
• Add example comparison plot to methods vignette