Function initialize_circle 

pub fn initialize_circle(
    height: usize,
    width: usize,
    center: (usize, usize),
    radius: f64,
) -> Vec<Vec<f64>>

Initialize a circle-shaped level-set function.

φ(x,y) = radius - √((x-cx)² + (y-cy)²) Positive inside the circle, negative outside.