Add Switzerland fetcher

[thiagovmdon]

Summary

This PR adds support for Switzerland to RivRetrieve Python.

It includes:

• a new SwitzerlandFetcher
• integration into rivretrieve/__init__.py
• documentation page for Switzerland
• cached station metadata
• example usage scripts
• tests and sample fixtures

Implementation details

• Metadata is retrieved from the public station endpoint:
  https://api.existenz.ch/apiv1/hydro/locations
• The main source website is:
  https://api.existenz.ch/#hydro
• For historical time series retrieval, the implementation uses the archive backend linked from the same source.
• Supported variables are:
  • discharge
  • stage
  • water temperature
• Both instantaneous values and daily means are supported.

Notes

• Cached metadata file currently contains 246 Swiss stations.
• Added a plotting example for quick manual verification of historical data.