Move tree building with NJ in a builder class

[junniest]

Move the tree building functionality into a builder class out of the Tree class.

Switch the nj_matrices.rs:70 back to setting branch lengths to 0.0 instead of f64::EPSILON since this change was only because of the python calculation breaking.

[MattesMrzik]

And also add a stochastic component to it. So dont take the min in argmin_wo_diagonal but make a weighted choice based on the distances. What would be a good function that transforms the row in the distance matrix to a probability vector?