Function sequential_integration::calculate_double_integral_simpson

pub fn calculate_double_integral_simpson(
    equation: &str, 
    first_integral_begin: f64, 
    first_integral_end: f64, 
    first_integral_step: f64, 
    second_integral_begin: &str, 
    second_integral_end: &str, 
    second_integral_step: f64
) -> Result<f64, Error>