Improve Cartopy CLI and add shared tutorial environment setup

[enikidis]

Summary

This PR makes the tutorial repository easier to run and publish by improving the Cartopy command-line tutorial and adding shared environment setup for the NetCDF and Cartopy tutorials.

Changes

• Refactor Cartopy/cera_cartopy.py into a clearer command-line workflow.
• Add CSV input validation for required columns: station_id, lat, and lon.
• Add Cartopy CLI options for:
  • --output
  • --no-show
  • --title
• Update Cartopy documentation with the expected CSV format and script examples.
• Add a shared Conda environment file: environment.yml.
• Add a pip fallback file: requirements.txt.
• Update the root README with setup and run instructions.
• Update the NetCDF and Cartopy tutorial READMEs to use the shared environment.
• Replace active notebook pip install cells with non-mutating setup guidance.
• Standardize notebooks on the Python (cera-tutorials) kernel.
• Add notebook-only checks to avoid re-downloading existing tutorial data files.
• Update .gitignore for generated files, local environments, downloaded sample data, and output images.

Testing

• Ran python3 -m py_compile Cartopy/cera_cartopy.py.
• Ran python3 -m py_compile Analyzing_NetCDF/NetCDF_Tutorial/net_CDF4.py.
• Validated both notebooks with jq empty.
• Verified no active notebook pip install commands remain.
• Ran git diff --check.
• Verified Cartopy runtime imports in the cera-tutorials Conda environment.
• Smoke-tested the Cartopy CLI commands:
  • python3 cera_cartopy.py water_level_stations.csv
  • python3 cera_cartopy.py water_level_stations.csv --output station_map.png
  • python3 cera_cartopy.py water_level_stations.csv --output station_map.png --no-show
  • python3 cera_cartopy.py water_level_stations.csv --title "Water level stations"

Notes

Downloaded tutorial data files remain local and ignored by git:

• Cartopy/water_level_stations.csv
• Analyzing_NetCDF/NetCDF_Tutorial/maxele.63.nc