Add function to import from TSP/VRP benchmark instances

[cmurray3]

In an effort to increase VeRoViz adoption, it would be nice to include a functionality to import some of the popular/classical TSP/VRP benchmark instances.

Many of these use (x,y) coordinates, so we'll need to include an option for the user to provide a lat/lon reference point (and some measure of the distance scale for the (x,y) coordinates.

This was inspired by #38 .

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Here are some places to start:

• cvrplib. Capacitated Vehicle Routing Problem Library, 2017. URL http://vrp.atd-lab.inf.puc-rio.br/index.php/en/
• tsplib. 2013. URL http://comopt.ifi.uni-heidelberg.de/software/TSPLIB95
• VRP-REP. 2017. URL http://www.vrp-rep.org/datasets.html.
• vrplib. VRPLIB Vehicle Routing Problem Library, 2020. URL http://or.dei.unibo.it/library/vrplib-vehicle-routing-problem-library.