Vector refactor

[IgorErin]

Proposed Changes

• DenseVector.toSparse refactor
• ClArray.map2*
• ClCell.toHostAnsFree
• ClArray.choose

Types of changes

What types of changes does your code introduce to GraphBLAS-sharp?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• Build and tests pass locally
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

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