spartialsortperm!!: partial sort perm function for sorting "top"-k of n with k<=n

[jacob-roth]

Add a function spartialsortperm!! that sorts the larges/smallest k elements of an n-array with k<=n. It's idea is to use a function sselect!! to do quickselect to find the kth element and then calls the existing SimultaneousSortperm.jl functionality to obtain the sorted vector and corresponding permutation.

I had a rough go at the spartialsortperm! and spartialsortperm variants but have commented them out since they are failing the tests.

One thing to note: for the testsets, I only require by.(v[p[1:k]])==by.(vref[pref[1:k]]) where v and p are the result of spartialsortperm!! and vref and pref are the result of Base's sortperm!, since there may be multiple "optimal" permutations. I noticed that your tests didn't do this before and am curious why you never encountered multiple "optimal" permutations for by=x->x÷100? Even after doing the second pass of sorting by index (unless Base.jl also does this?)

Related to: #1 (comment)