Add Netherlands

[thiagovmdon]

Summary

Related to issue #97

This PR adds Netherlands support to RivRetrieve via a new NetherlandsFetcher backed by Rijkswaterstaat WaterWebservices.

What was added

• new fetcher: rivretrieve/netherlands.py
• package export in rivretrieve/__init__.py
• documentation page: docs/fetchers/netherlands.rst
• API docs entry in docs/api.rst
• cached metadata catalog: rivretrieve/cached_site_data/netherlands_sites.csv
• example usage script: examples/test_netherlands_fetcher.py
• tests and fixtures for metadata, discharge, stage, and temperature retrieval

Supported variables

• discharge_daily_mean
• discharge_instantaneous
• stage_daily_mean
• stage_instantaneous
• water-temperature_daily_mean
• water-temperature_instantaneous

Implementation notes

• uses Rijkswaterstaat catalog and observation services
• prefers the current ddapi20 endpoint family and falls back to the legacy service if needed
• filters metadata to stations supporting at least one RivRetrieve-supported surface-water variable
• aggregates sub-daily observations to daily means for daily products
• keeps native timestamps for instantaneous products
• converts stage from cm to m
• converts discharge from m3/d to m3/s when needed
• filters provider sentinel missing values from temperature series before aggregation