Add non-gridded data notebooks

[npedrazzini]

This PR introduces the following changes:

1. notebook extract_anomalies.ipynb, which
   • downloads daily mean sea level pressure (psl) data (2014–2020) using the download_toolbox
   • downloads monthly average psl data (1979–2020) via the CDS API
   • calculates climatologies from the monthly data
   • computes daily psl anomalies using the calculated climatologies
2. notebook degrid_and_visualise.ipynb, which
   • demonstrates how to generate synthetic station and buoy datasets from gridded data:
     • stations: extracts N random lat–lon points and interpolates their values from the surrounding grid over the full period
     • buoys: extracts N random points and simulates movement by finding nearest neighbours in subsequent time steps, adding positional jitter to mimic real‑world buoy drift
3. Conda environment config (nongriddedenv.yaml)
   • adds a conda environment YAML file to run notebooks

Closes #23

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Coverage report

This PR does not seem to contain any modification to coverable code.

[jemrobinson]

I've looked through these notebooks - they look fine but I haven't actually run them.

[IFenton]

A couple of minor comments, but apart from that looking fine, and ready to merge.