Support for models with gfam() family

[gavinsimpson]

Add support for models fitted with family = gfam(). These models can be fitted by

• gam()
• bam()

with any non-matrix response using the base R families or the extended families.

Support need for

• data_sim() #266
• gfam support in appraise() #267
• draw() works with gfam models #268
• gfam support in fitted_samples() #269
• gfam support in posterior_samples() #270
• gfam support in smooth_samples() #271
• gfam support in simulate.gam() #272