Tracking unexported brms functions

[athowes]

We use four brms functions:

• brms:::dpar_bounds
• brms:::validate_data
• brms:::validate_family
• brms:::validate_formula

See:

N  checking dependencies in R code (2.2s)
   Unexported objects imported by ':::' calls:
     ‘brms:::dpar_bounds’ ‘brms:::validate_data’ ‘brms:::validate_family’
     ‘brms:::validate_formula’
     See the note in ?`:::` about the use of this operator.

This issue is a place to track progress on removing dependence on these functions, or them being exposed within brms. Adam to link issue raised in brms repository about this.

[athowes]

Adam to link issue in brms.

[athowes]

paul-buerkner/brms#1676

[seabbs]

brms:::log_lik_weight and all internal family log_like and posterior predict functions

[athowes]

Note that this issue blocks CRAN submission.