kvr2: Calculate and Compare Multiple Definitions of Coefficient of Determination

Calculate nine types of coefficients of determination (R-squared) based on the classification by Kvalseth (1985) . This package is designed for educational purposes to demonstrate how R-squared values can fluctuate depending on the choice of formula, particularly in power regression models or linear models without an intercept. By providing a comprehensive list of definitions, it helps users understand the mathematical sensitivity of goodness-of-fit indices.