Deprecate HYDRO min_power in favour of min_flow

[lbonaldo]

Description

This PR introduces a deprecation path for the HYDRO min_power column and function, replacing them with min_flow. This change improves code consistency, since the column is actually used to enforce a minimum outflow requirement (power generation + spillage).

Changes Made

1. Function Deprecation (src/model/resources/resources.jl)
   Deprecated: min_power(r::Hydro) function with deprecation warnings
   Added: New min_flow(r::Hydro) function that provides the same functionality
   Backward Compatibility: Old function continues to work during transition period
   Smart Fallback: Automatically detects and uses the appropriate column (min_flow preferred, min_power as fallback)
2. Hydro Module Update (src/model/resources/hydro/hydro_res.jl)
   API Migration: Updated hydro constraints to use the new min_flow API
3. Tests (test/test_load_resource_data.jl)
   API Validation: Confirms new min_flow function works with updated column names
4. Documentation (docs/src/User_Guide/model_input.md)
   Deprecated: Min_Power column is kept with deprecation note
   Added: New Min_Flow column is documented

Migration Guide

For Users:

Immediate: No action required - existing code continues to work
Recommended: Update input data to use Min_Flow column instead of Min_Power
Future: min_power function will be removed in a future breaking release

For Developers:

New Code: Use min_flow(r::Hydro) instead of min_power(r::Hydro)
Data Files: Rename Min_Power columns to Min_Flow for clarity

Breaking Changes

None: This is a non-breaking change with full backward compatibility
Future: min_power function will be removed in a future breaking release

What type of PR is this? (check all applicable)

• Code Refactor

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and .md files under /docs/src have been updated if necessary.
• The latest changes on the target branch have been incorporated, so that any conflicts are taken care of before merging. This can be accomplished either by merging in the target branch (e.g. 'git merge develop') or by rebasing on top of the target branch (e.g. 'git rebase develop'). Please do not hesitate to reach out to the GenX development team if you need help with this.
• Code has been tested to ensure all functionality works as intended.
• CHANGELOG.md has been updated (if this is a 'notable' change).
• I consent to the release of this PR's code under the GNU General Public license.

How this can be tested

By running the test suite.

Post-approval checklist for GenX core developers

After the PR is approved

• Check that the latest changes on the target branch are incorporated, either via merge or rebase
• Remember to squash and merge if incorporating into develop

[sambuddhac]

Looks good. Approved !!!

[gschivley]

Very minor, but can you change because the model might not be able to generate to because the unit might not be able to generate?

[lbonaldo]

of course!