Provide general block spmv interface and specializations

[mzuzek]

• Implement general spmv() interface that specializes for block and non-block crs inputs (matrix template).
  • see issue Implement general spmv() interface that specializes for block and non-block crs inputs (matrix template). #10 and PR Create new interface for spmv  #13
• Explore specialization options templating on alpha, beta, N/T/C/H mode, etc. (similar to non-block spmv() - see the chart on Figure out the specialization of template function spmv #2 (comment)):
  • implement Conjugate where appropriate + include std::complex matrices in test samples (see follow up Support std::complex #27);
  • add tests to cover corner cases (esp. alpha/beta=0/1/*) to existing N/T/C/H (note: no need for cross combining);
  • split source files if helpful:
    • see issue Implement general spmv() interface that specializes for block and non-block crs inputs (matrix template). #10 and PR Create new interface for spmv  #13
  • check controls (e.g. dynamic/static schedule) implementation and supply missing items;
  • fold block specializations (be careful about blockSize=1 performance): PR #9 Refactoring of explicit expansion for small block size #18
    • check if we can extract underlying crs matrix and run regular spmv for blockSize=1; moved to Optimize blockSize=1 specializations #28