Add toml parsing for parameters

[ajz34]

This PR will introduce parsing toml dictionary to define DFTD3 parameters, and also allow the API caller to retrive DFTD3 parameters in struct DFTD3DampingParam.

The minimal full example can be

use dftd3::prelude::*;
// Use custom parameters by toml string
let input = r#"{version = "d3bj", a1 = 0.3981, s8 = 1.9889, a2 = 4.4211, atm = false}"#;
// You can also use the following input to specify B3LYP-D3(BJ) parameters
// let input = r#"{version = "d3bj", method = "b3lyp"}"#;
// toml parameter type
let damping_param = dftd3_parse_damping_param_from_toml(input);
// FFI parameter type
let dftd3_param = damping_param.new_param();

let atom_charges = vec![8, 1, 1];
// coordinates in bohr
#[rustfmt::skip]
let coordinates = vec![
    0.000000, 0.000000, 0.000000,
    0.000000, 0.000000, 1.807355,
    1.807355, 0.000000, -0.452500,
];
let model = DFTD3Model::new(&atom_charges, &coordinates, None, None);
let res = model.get_dispersion(&dftd3_param, false);
let eng = res.energy;
println!("Dispersion energy: {eng}");