More useful printed result for a single parameter

[sethaxen]

Currently, if we use PSIS for a single parameter, we get the following output:

PSISResult with 1 parameters, 1000 draws, and 1 chains
Pareto shape (k) diagnostic values:
                    Count       Min. ESS 
 (-Inf, 0.5]  good  1 (100.0%)  363

The downside here is that the actual shape value is not shown, and the Count column is not informative. Probably something like this would be more useful:

PSISResult with 1 parameters, 1000 draws, and 1 chains
Pareto shape (k) diagnostic value:
                   Shape k  ESS 
good  (-Inf, 0.5]  0.3      363

OR

PSISResult with 1 parameters, 1000 draws, and 1 chains
    Shape diagnostic k: 0.3 ∈ (-Inf, 0.5]  (good)
    ESS: 363