Add S3 methods for model fit and model class classes

[samuel-watson]

Add a range of S3 methods for the Model class and for the model fit outputs.

• [x] summary
• [x] coef (print the coefficients)
• [x] family (return the family object)
• [x] formula (return the formula)
• [x] vcov (print instructions to return different matrices - Sigma or information matrix, or D)
• [x] predict (print instructions on prediction)
• [x] fitted
• [x] residuals
• [x] logLik (log lik object)
• [x] confint (model fit)