add game: squadro

[giogix2]

Hi all.

I've added the implementation of a game called Squadro. It's a two-player, perfect information game. I believe it has some similarities with Quoridor, which is already present in open_spiel. Just like Quoridor, it's distributed by Gigamic. Though, I'm not sure whether it's protected by copyright.

It doesn't have a Wikipedia page, but you can find it here in boardgamegeek https://boardgamegeek.com/boardgame/245222/squadro.

Hopefully, this is helpful and can be added to the collection of games.

Best,
Gio

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[lanctot]

Hi @giogix2, thanks!

First thing, can you sign the CLA? (See the "Details" link above and follow the instructions.)

[giogix2]

Hi @lanctot. Signed.
There was also an issue with the contributor's name. Also fixed that.