Add the ability to select a subpart from dataset

[toodef]

Frequently we need to extract part of dataset for test network for example.
We need to implement this feature in DataProducer, cause it'll make possible to not change dataset class. Also, DataProsucer has flushing and loading indices interfaces.

Requirements:

• Possible of flushing indices for reproducible
• Different strategies for indices selection: (from begin, from range, all of it with step, e.t.c)