Generalize istriu/istril to accept a band index

[jishnub]

Currently, only istriu(S) and istril(S) are specialized for sparse matrices, and this PR generalizes these to accept the band index k. This improves performance:

julia> S = sparse(1:4000, 1:4000, 1:4000);

julia> @btime istriu($S, -1);
  20.941 ms (0 allocations: 0 bytes) # main
  5.660 μs (0 allocations: 0 bytes) # this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.07%. Comparing base (780c4de) to head (598da1f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #590   +/-   ##
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  Coverage   84.07%   84.07%           
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  Files          12       12           
  Lines        9188     9188           
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  Hits         7725     7725           
  Misses       1463     1463           

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[dkarrasch]

Nice!