Add Kling-Gupta Efficiency

[jarq6c]

This PR adds the Kling-Gupta Efficiency hydrological model calibration/evaluation metric.

Gupta, H. V., Kling, H., Yilmaz, K. K., & Martinez, G. F. (2009). Decomposition of the mean squared error and NSE performance criteria: Implications for improving hydrological modelling. Journal of hydrology, 377(1-2), 80-91.

Additions

• hydrotools.metrics.metrics.kling_gupta_efficiency
• hydrotools.metrics._validation

Removals

• None

Changes

• None

Testing

1. Added test_kling_gupta_efficiency to test_metrics
2. Added test_validation to check input validation methods

Notes

• Closes #167

Todos

• Implement later refinements to this metric, as needed.

Checklist

• [x] PR has an informative and human-readable title
• [x] PR is well outlined and documented. See #12 for an example
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows project standards (see CONTRIBUTING.md)
• [x] Passes all existing automated tests
• [x] Any change in functionality is tested
• [x] New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
• [x] Placeholder code is flagged / future todos are captured in comments
• [x] Reviewers requested with the Reviewers tool :arrow_right:

[jarq6c]

Not only does it look good but it showed an error in some of my other code. And... approved.

Thanks for the review!