CUSPARSE BSR improvements

[maleadt]

• make nnz return the number of nonzero elements, and not the number of nonzero blocks.
• add indexing support
• fix BSR to CSR conversion

[codecov]

Codecov Report

Merging #1409 (799faa9) into master (a50e85c) will decrease coverage by 0.07%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1409      +/-   ##
==========================================
- Coverage   77.05%   76.97%   -0.08%     
==========================================
  Files         124      124              
  Lines        9186     9209      +23     
==========================================
+ Hits         7078     7089      +11     
- Misses       2108     2120      +12     

Impacted Files	Coverage Δ
lib/cusparse/array.jl	64.98% <45.45%> (-1.69%)	⬇️
lib/cusparse/conversions.jl	81.48% <100.00%> (-0.08%)	⬇️
lib/cusparse/level2.jl	90.32% <100.00%> (ø)
lib/cusparse/level3.jl	75.00% <100.00%> (ø)
lib/cupti/error.jl	66.66% <0.00%> (-3.34%)	⬇️
lib/cutensor/error.jl	41.17% <0.00%> (-2.58%)	⬇️
lib/cufft/error.jl	38.88% <0.00%> (-2.29%)	⬇️
lib/utils/memoization.jl	86.27% <0.00%> (+0.27%)	⬆️
lib/cusparse/error.jl	94.73% <0.00%> (+0.29%)	⬆️
... and 5 more

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