eeg_topoplot labels fix

[behinger]

As of now, the labels in the eeg_topoplot have not every worked (strange, but oh well..).

In any case, I fixed this now, but I also moved labels from a positional argument to a keyword argument. I think this makes much more sense, you can either specify positions, labels or both.

One argument could be made to make positions a positional argument, like in topoplot itself and overload it, but I didnt manage with convert argument, and I think this way it is ok.

This is a breaking change.

PS: I might want to adress few other issues which might be breaking too (e.g. #38), so please hold with new version