verbose, display_warnings etc

[MKul]

First - amazing job with this package!
It would be quite nice if the hole package could have parameter verbose that switch off all messages except errors - for now it is possible to switch off a part of it, but e.g. public_data_interface and private_data_interface have display_warnings parameter in some of the functions that is fixed and can not be altered during initialization of Data object. I've also noticed that you mix print and logging - if you switch entirely to logging the above request would be quite easy to implement by logging level.