make client sampler class

[fvarno]

There are different ways of sampling clients. Currently we pass the sampling scheme and rate directly as algorithm arguments. It is better to make client sampler a seperate class and an instance is made and passed to the algorithm as argument instead.

[fvarno]

An alternate solution is to create a ClientManager class and move the _client memory definition there. It would be more interesting if DataManager also supported assigning clients to partition indices (Mapping). The assignment could be defined by a ClientManager class's public API method. Thus, if the number of clients is less than the number of partitions, the selected partitions can be drawn in any predetermined order.