apollon.som.utilities.distribute should handle 2d indices

[Teagum]

BMUs can only be passed as 1d index. This results in a mapping from these index to the matching input data. However, users cannot simply infer the map position form the index.

Possible solutions:
* dynamically convert the 1d index to a tuple base on the value of kwarg
* allow 1d and 2d data to be passed in