Refactor handlers

[pelesh]

This PR features refactored handler classes. The changes are as follows:

• Memory space of the handler is set upon instantiation.
• Host handler accompanies each device handler.
• Selection between device and host handler is done in the same way as for matrix/vector classes.
• All examples updated.
• All tests updated and passing.
• Closes getRandSketchingMethod should be called setRandSketchingMethod #99.