Re-add `print.compare.loo(simplify=)`

[mattansb]

Previous version of {loo} had the option to show full ELPD info for the compared models, but this is now lost:

library(loo)
#> Warning: package 'loo' was built under R version 4.5.3
#> This is loo version 2.9.0.9000
#> - Online documentation and vignettes at mc-stan.org/loo
#> - As of v2.0.0 loo defaults to 1 core but we recommend using as many as possible. Use the 'cores' argument or set options(mc.cores = NUM_CORES) for an entire session.
#> - Windows 10 users: loo may be very slow if 'mc.cores' is set in your .Rprofile file (see https://github.com/stan-dev/loo/issues/94).
LL <- example_loglik_array()
loo1 <- loo(LL)     # should be worst model when compared
loo2 <- loo(LL + 1) # should be second best model when compared
loo3 <- loo(LL + 2) # should be best model when compared

comp <- loo_compare(loo1, loo2, loo3)

All the info is there, but not shown in print:

names(comp)
#>  [1] "model"       "elpd_diff"   "se_diff"     "p_worse"     "diag_diff"  
#>  [6] "diag_elpd"   "elpd_loo"    "se_elpd_loo" "p_loo"       "se_p_loo"   
#> [11] "looic"       "se_looic"

print(comp, simplify = FALSE)
#>   model elpd_diff se_diff p_worse diag_diff diag_elpd
#>  model3       0.0     0.0      NA                    
#>  model2     -32.0     0.0    1.00   N < 100          
#>  model1     -64.0     0.0    1.00   N < 100
#> 
#> Diagnostic flags present.
#> See ?`loo-glossary` (sections `diag_diff` and `diag_elpd`)
#> or https://mc-stan.org/loo/reference/loo-glossary.html.

^{Created on 2026-06-06 with reprex v2.1.1}