change special treatment for `unif` distributions

[davidchampredon]

The code contributed for version 1.6.0 treats the unif distribution differently than the other ones regarding the sampling of distribution families. This was part of hot fixes but that should be changed such that the unif family of distributions is consistant with the other ones (e.g., gamma).

Suggestion: the lower and upper bound of the uniform distribution should each have their own mean and sd in their definition. For example the definition should look like:

foo = list(dist = "unif", mean_lower = 1, mean_upper = 9, sd_mean_lower = 0.2, sd_mean_upper = 0.3)