Allow alphanumerical group ids

[Sebobo]

Currently setGroup requires an integer, which will be set as label of the resulting cluster.
But it's more helpful to have alphanumerical cluster names at the end.

Therefore I currently do a string replace on the unique integer id but it would be nice to just set the group id as string.

[clue]

@Sebobo Thanks for your input, I think you're raising a good point. There's currently some discussion to remove this whole feature in the underlying library, perhaps you can join the discussion in graphp/graph#168? 👍