Add OpenRouteService (ORS) as backend

[sfendrich]

Add support for ORS as a backend for matrix and route computations.

[jcoupey]

Related to the discussion in #192.

[jcoupey]

@sfendrich thanks for the PR! I'll see if I can manage to setup an ORS server locally to test this out.

[sfendrich]

If you have any problems with the ORS setup, don't hesitate to ask me for help.

[jcoupey]

@sfendrich thanks, I fiddled a bit with errors with the maven build but managed to get things going using docker.

[jcoupey]

I tested the branch locally and everything works fine. I just have a couple comments/questions in the PR review.

I was wondering how ORS deals with absent values in the matrix, but it turns out the existing null-handling code works out of the box (handy for example for points outside the dataset). I also checked plotting solutions through the frontend and everything is fine with the encoded geometry.

@sfendrich could you also mention the support for ORS as a routing engine in CHANGELOG.md?

[sfendrich]

ORS also returns null if no route can be found.

I'll update the changelog as requested.

[jcoupey]

I just merged #204 so this is currently available in master and will be part of the next release.