Validates the family/sigma combination and returns a list with:
sigma_fun: NULL (fixed-sigma path) or a closure
function(eta) returning sigma at each eta.
family: the family object (possibly set to NULL for
gaussian(identity) short-circuit).
resolve_sigma_family(family, sigma_missing, sigma = NULL, offset = 0)A list with components sigma_fun and family.
A family object or NULL.
Logical: was sigma missing in the calling function?
Numeric sigma value (only used when sigma_missing is FALSE).
Numeric scalar offset.