Add PCHIP interpolation

[david-bentley]

Description

Exposes PCHIP (Piecewise Cubic Hermite Interpolating Polynomial) interpolation, just calling down into RegularGridInterpolator from scipy.

I'm not sure that adding into the generic interpolator is the best place for this, as I think the use case is really more for vertical interpolation. But proposing to generate discussion.

As an example, here is a vertical wind profile interpolated using linear (black) vs PCHIP (blue):

The motivation is to avoid the discontinuities in (the derivative of) the wind profile.

Checklist

Important

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• Included a What's New entry
• Incorporated type hints in any changed code
• Checked if tests need updating
• Checked if benchmarks need updating
• Checked if documentation needs updating
  • Docstrings (we love code examples!)
  • User Manual pages
• Checked if dependencies need updating
• Confirmed that the GitHub 'checks' on this PR are passing ✅
  (further reading)

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