Add structured FETCH range calculation modules

[MaleenKidiwela]

Motivation

• Refactor the procedural FETCH Range Calculation.ipynb analysis into reusable, unit-focused Python modules to improve maintainability and reuse.
• Preserve the original notebook workflow and file naming while enabling programmatic usage from scripts and notebooks.
• Provide explicit helpers for salinity calibration, tidal correction, velocity interpolation, and baseline/tilt handling so downstream analyses can be composed and tested.

Description

• Add modular implementations extracted from the notebook: code/range_salinity.py, code/tidal_correction.py, code/velocity_interpolation.py, code/pressure_moving_average.py, code/range_calculation_workflow.py, and code/range_calculation_main.py, plus a notebook scaffold code/fetch_range_calculation.ipynb that demonstrates the workflow.
• Implement salinity calibration (gap stitching, post-gap smoothing, bottle calibration) in range_salinity.py and expose SalinityCalibrationConfig, GapMeanShiftConfig, prepare_salinity_series, and bottle_residuals.
• Implement tidal-prediction parsing, optimization and pressure correction in tidal_correction.py, velocity interpolation in velocity_interpolation.py, and 15-day pressure moving-average helpers in pressure_moving_average.py.
• Add build_baseline_perturbations to code/positioning.py to construct per-baseline perturbation timeseries, export the new helpers from code/__init__.py, and document modules in code/README.md.

Testing

• No automated tests were run for this change.

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Codex Task