Download instances and solutions locally

[leonlan]

Downloading an instance currently does not save the instance locally. We should rewrite the code to actually download the files to save them locally.

The user would then have to write this:

import vrplib

# Download the instance and solution locally
vrplib.download_instance("X-n101-k25", "/path/to/X-n101-k25.vrp")
vrplib.download_solution("X-n101-k25", "/path/to/X-n101-k25.sol")

# Read the downloaded instance and solution
instance = vrplib.read_instance("/path/to/X-n101-k25.vrp")
solution = vrplib.read_solution("/path/to/X-n101-k25.sol")