Add a function to extract the (inverse) link function from an estimated GAM

[gavinsimpson]

It is useful to be able to extract the (inverse) link function from the family object stored in an estimated GAM.

Need:

• [x] inv_link() for the inverse of the link function
• [x] link() for the link function
• [x] family method for objects of class
  • [x] "gam"
  • [x] "gamm"
  • [x] "bam"
  • [x] "glm"
  • [x] (handle gamm4() objects?)

Will need an argument parameter where the user can specify which parameter of the response distribution is required as we want to handle GAMLSS or distributional models. Default parameter is "mu".

Perhaps allow "location", "scale", "shape" as options to parameter as those are more generic in terms of the GAMLSS terminology.

[gavinsimpson]

As of 72954e8, all the currently-known families in mgcv are now covered by extract_link()