Remove hydrostats and extraction functionality from HAT

[andreas-grafberger]

Description

Up until now, HAT includes components, including these two:

• Time-series extraction at stations
• Computation of error metrics for hydrological verification

Both functionalities will no longer live in HAT but were moved to https://github.com/ecmwf/hyve.

This PR removes these features and associated documentation.

NOTE: Given the changed scope of HAT, we'll need to rewrite our documentation and change the structure of the codebase. This will be done in follow up PRs.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.