Function write_bedgraph_streaming 

pub fn write_bedgraph_streaming<P>(
    path: &Path,
    produce: P,
) -> Result<(), Box<dyn Error>>
where
    P: FnOnce(&mut dyn FnMut(&str, usize, f64) -> Result<()>) -> Result<()>,

Write a bedGraph, pulling scores from produce as they are needed.

Nothing is retained: each score is formatted and written as it arrives.