Make hpd for AbstractVector public

[itsdfish]

Hello,

I want to inquire about the possibility of making hpd for AbstractVector a public method. I would like to use it for analyzing generated quantities. Currently, it is a private method.

[sethaxen]

I would suggest waiting on this until #430 is completed, as this will make some breaking changes to hpd, deprecate it, and replace it with PosteriorStats.hdi

[itsdfish]

I was not aware of PosteriorStats.jl. I will use that instead.