Provide option to generate TS file through the console interface

The file demo_data_region_16s_2048Hz.npy is needed in multiple places, both in notebooks and main functions we provide for console viewers. It can be generated through a function (generate_region_demo_data) that is currently present only in one notebook, it should be moved to a more common place.

This has been discussed in PR #457 and the conclusion is:
"I suppose the generate_region_demo_data.py used to generate the same dataset (demo_data_region_16s_2048Hz.npy). I would suggest keeping only the function and run it automatically when the dataset is not found.
Would it be better to make the generate_region_demo_data function available through the tvb.simulator.lab interface?"