DataModeler: Get coefficient of determination

Get the coefficient of determination (R2) of the model. The R2 goodness of fit is a number between 0 and 1 that measures how well a model predicts the data. A value of 1 means a perfect fit. If all dependent data, the y values, have the same variance the R2 is simply 1 - SSmodel/SStotal, where SSmodel is the sum of the squared differences between y's predicted by the model and the actual y values from the data, and SStotal the sum of the squared differences between the actual data y values and the mean of the data.

Because our data points can have different weights we use adapted formula from Buse (1973).