Refactor `Sample` implementation: avoid `Tables.rowtable`

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a48458a) 94.26% compared to head (c21ed50) 94.28%.

Files	Patch %	Lines
src/transforms/sample.jl	88.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #235 +/- ## ========================================== + Coverage 94.26% 94.28% +0.01% ========================================== Files 41 41 Lines 1255 1259 +4 ========================================== + Hits 1183 1187 +4 Misses 72 72 ```

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