Sort only upper tail weights

[sethaxen]

Fixes #20.

Benchmarks:

julia> using PSIS

julia> x1 = randn(1_000);

julia> x2 = randn(100, 1_000, 4);

Before this PR:

julia> @btime $psis($x);
  89.436 μs (7 allocations: 16.75 KiB)

julia> @btime $psis($x2);
  16.521 ms (1144 allocations: 6.24 MiB)

After this PR:

julia> @btime $psis($x);
  71.606 μs (10 allocations: 16.86 KiB)

julia> @btime $psis($x2);
  7.827 ms (1444 allocations: 6.25 MiB)

[codecov-commenter]

Codecov Report

Merging #22 (ba6bf0e) into main (ae98bab) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files           8        8           
  Lines         307      307           
=======================================
  Hits          298      298           
  Misses          9        9           

Impacted Files	Coverage Δ
src/core.jl	97.54% <100.00%> (ø)

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