Greece OHI

[thiagovmdon]

Related to issue #106

Description

This PR adds a Greece fetcher based on the OpenHI / Enhydris public API. It follows the same overall architecture and project conventions as the recent country fetchers, including cached station metadata, example usage, detailed Sphinx documentation, and unit tests.

Summary

• add GreeceFetcher for the OpenHI / Enhydris public API
• support stage_daily_mean, stage_instantaneous, discharge_daily_mean, and discharge_instantaneous
• export the fetcher from rivretrieve.__init__
• add cached Greece station metadata, detailed Sphinx docs, example usage, and unit tests

Implementation details

• discover hydrometric stations from the union of ts_only: variable:stage, ts_only: variable:level, and ts_only: variable:discharge
• normalize provider Stage and Water Level to RivRetrieve stage variables
• normalize stage units from cm to m
• convert provider sentinel values such as -999, -9999, and -6999 to missing values
• rank compatible groups and timeseries deterministically, preferring checked/public series and falling back when aggregated daily series are empty
• document that cached provider-derived metadata remain subject to the OpenHI CC BY-SA 4.0 licence