Add flat-slice dataset access (getDataFlat with offset)

[jamesmudd]

Motivation

• Provide an API to read a flattened (1D) view of a sliced dataset so callers can efficiently request a contiguous flattened slice without post-processing.

Description

• Add Object getDataFlat(long[] sliceOffset, int[] sliceDimensions) to the Dataset interface.
• Implement flat-slice reading in DatasetBase by validating the slice, obtaining the slice buffer, computing total elements, and delegating to DatasetReader.
• Reject flat-slice requests for writable datasets by throwing HdfWritingException in WritableDatasetImpl to match existing slicing behavior.
• Add tests in SlicingTest to verify flat-slice correctness and error cases for empty and scalar datasets.

Testing

• Ran ./gradlew test --tests io.jhdf.dataset.SlicingTest and all tests in that suite passed (build completed successfully with minor Gradle configuration warnings).

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Codex Task

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud