Add unit_diag option for sv2! functions

[amontoison]

ilu02 function returns a matrix composed of a lower triangular and a unit upper triangular factor. I updated sv2! to work with unit triangular matrices.
Should I use unit_diag as an argument with default value false or keep it as a keyword argument?

I also found that istril has missing methods (JuliaLang/julia#38404), I updated the code to avoid scalar operations.

[codecov]

Codecov Report

Merging #540 (2d7180f) into master (e09bf71) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
- Coverage   77.83%   77.82%   -0.01%     
==========================================
  Files         116      116              
  Lines        6415     6423       +8     
==========================================
+ Hits         4993     4999       +6     
- Misses       1422     1424       +2     

Impacted Files	Coverage Δ
lib/cusparse/interfaces.jl	37.14% <66.66%> (+3.80%)	⬆️
lib/cusparse/level2.jl	83.14% <78.57%> (+0.58%)	⬆️

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[maleadt]

Thanks!
A keyword argument seems fine; this is an internal wrapper anyway, so we can always update it in the future.