Fix aet, replaces #142 (#143)

* chore: small formatting refactor

Co-authored-by: ajkhattak <ajkhattak@gmail.com>

* feat: add method to compute et_from_retention_depth

Co-authored-by: ajkhattak <ajkhattak@gmail.com>

* fix: take AET from surface retention depth

Co-authored-by: ajkhattak <ajkhattak@gmail.com>

* fix: try to prevent segfault when computing AET from retention depth

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Co-authored-by: ajkhattak <ajkhattak@gmail.com>

Release version 2.0.0

CFE1.0 uses GIUH-based surface runoff scheme.