Implement start position read API (issue #23)

[kiloconnect]

Summary

This PR implements the start position read API as outlined in the implementation plan (issue #23). It adds the ability to read channel data starting from a specific sample position, with optimizations for skipping entire data blocks.

Changes

New API

• Added read_channel_from(channel, start, output) method to TdmsFile
• Refactored existing read_channel() to use read_channel_from() internally for consistency

Implementation Details

• Added read_single_from() and read_from() methods to DataBlock
• Extended MultiChannelContigousReader with read_from() for offset support
• Extended MultiChannelInterleavedReader with read_from() for offset support

Performance Optimizations

• Block-level skipping: Entire data blocks are skipped when the start position is beyond them
• Contiguous layout: Seeks directly to the offset within channel data
• Interleaved layout: Skips entire rows before reading

Testing

• Added comprehensive unit tests in tests/read_with_offset.rs
• Tests cover:
  • Reading from position 0 (should match normal read)
  • Reading from middle of data
  • Reading with small output buffers
  • Reading beyond available data
  • Reading at various positions
  • Backward compatibility

Backward Compatibility

The implementation maintains full backward compatibility:

• The original read_channel() method signature is unchanged
• Existing code will continue to work without modifications
• The new functionality is opt-in via the new read_channel_from() method

Related Issues

Closes #23

Testing

All existing tests should pass, and new tests have been added to verify the offset reading functionality.

Documentation

The new method includes comprehensive documentation explaining:

• Parameters and their purpose
• Performance characteristics
• Usage examples in the doc comments