Closes #31. Add \iter\unique which returns an iterator with unique va…

[alexkart]

Closes #31 . Add \iter\unique which returns an iterator with unique values.

[alexkart]

Sorry, specified wrong issue in the commit message (#37 instead of #31)

[Neirda24]

Tell me if I'm wrong but this implementation goes against all advantages of using iterator (memory safe). Because in order to check the unicity we build up a big array in memory.... I don't see an implementation that would be valid though....

[drupol]

This specific operation is indeed not very optimal because it needs to go through the whole iterator.