Always include `p_outlier` in `make_hssm_rv`

Just always append `p_outliers` to `list_params` no matter whether the user specifies it or now, and if it shows up anywhere other than the last position in `list_params`, throw an error

Start by locating the make_hssm_rv entry point and reading how it builds list_params. Confirm the requested behavior for p_outliers, including rejecting it when it is not last; done means the function consistently appends it when absent and raises an error for an invalid position.

Written by the indexing model from the issue text.