Debug implementation for Smartcore structs in notebooks

[Mec-iS]

Common structs should have implemented their own evcxr_display function in Debug trait to be visualized correctly in Notebooks (as HTML).

For example as found here for List:

use std::fmt::Debug;
pub struct List<T>(Vec<T>);
impl<T: Debug> List<T> {
    pub fn evcxr_display(&self) {
        let mut html = String::new();
        
        html.push_str("<ol>");
        
        for item in &self.0 {
            html.push_str(&format!("<li>{:?}</li>", item));
        }
        
        html.push_str("</ol>");
        println!("EVCXR_BEGIN_CONTENT text/html\n{}\nEVCXR_END_CONTENT", html);
    }
}

Different issues for this can be opened in main smartcore repo.