Allow info to be stored per node in an array of node ids

[pankajgupta]

Motivated by #138 where a centrality score is being computed per node and it might be sometimes most efficient to keep information per node stored in an array.

Currently we have graph.tourist.InfoKeeper that keeps info in a mutable.Map which precludes storing infoPerNode in an array.

How about declaring a new trait that has get/put. and allowing infoPerNode to return a collection of this trait type. This can be either a mutable.Map (as now) or an Array with implicit conversions from the Map and Array to this trait?

Comments @szymonm ?