Support precision casting for DScalar

[AdamGS]

This is a missing piece to support #5863

[codecov]

Codecov Report

❌ Patch coverage is 98.38710% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.39%. Comparing base (5cd8b29) to head (4f689ea).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-compute/src/cast/dvector.rs	98.38%	2 Missing ⚠️

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