Custom families

[lindeloev]

Family-specific functions are currently scattered over several locations in the code and dealt with in an if-else like fashion. This hard-coding of response families makes it harder to implement new ones and maintain existing ones.

I propose storing all of this in an mcpfamily() object which extends gaussian(), etc.

• Move priors to mcpfamily()
• Move the JAGS likelihood code to mcpfamily()
• Move R random generator code to mcpfamily() (used in fit$simulate()).
• Assign a list of distributional parameters (dpar) for each family, e.g., c("mu", "sigma") for gaussian() and build regression models for each of these. Use intercept-models for all that are not explicitly included in the formulas.
• Also somehow code whether it supports ar(), weight(), etc.
• Document it in a vignette.

This is relevant for #89. Once implemented, support for stan will also be easier (#100).