Add concept: `sets`

[sanderploegsma]

The goal of this issue is to add a new concept to the Java track that introduces sets. The corresponding concept exercise can be probably be based on one the following exercises from other tracks:

• Gotta Snatch 'Em All from the Elm track
• Ozan's Playlist from the Javascript track

Instructions on how to contribute to the Java track and how to implement concept exercises can be found in the contributing guide.

As always, leave a comment here if you want to work on this exercise, and we'll assign it to you. In case you have any further questions, feel free to ask here.

[okaram]

@sanderploegsma I can work on this one, if it's OK ...

[sanderploegsma]

Great! I'll assign you to it.

Make sure to check out the contributing guide in this repository if you haven't already, and feel free to comment here in case you have any questions!

[sanderploegsma]

After adding the concept, it should be added to the prerequisites in the config.json for the following practice exercises:

• Custom Set
• Go Counting
• Robot Name
• Saddle Points

[sanderploegsma]

@okaram I'm unassigning you from this issue for now so that others can pick it up if they want to. If you still want to contribute please drop a comment and we can assign it back to you!