Add anti-van der Waerden Numbers

[derrickstolee]

For integers k and n, the anti-van der Waerden number aw(k,n) is the minimum t such that every t-coloring of {1,...,n} (that uses each color at least once) contains a k-term arithmetic progression whose elements have distinct colors (a rainbow k-AP). These numbers have connections with Szemeredi numbers.

See Rainbow arithmetic progressions for more.

Algorithms from derrickstolee/RainbowAPs can be adapted to this repo.