Don't initialize devices explicitly in doctests.

Don't initialize devices explicitly in doctests.

Rely on the internal `.load_from_config()` methods to do that for us.
This reduces duplication (e.g. device filename only set in config
defaults, not in doctests too).  It also makes it posible to use
several devices to build your AFM interface, where the old doctests
would only work with all the communication going through a single
card.