Add SelectionValue to the base

to access the iterable stored by Selection.
Add a dummy SeletionDevice

and use in tests
Add SelectionValue iterable access test
Use proper list for specifying ranges in scan
Clarify scan_param_feedback in the docstring
Add callbacks for every parameter range in scan
Test scan callbacks
Provide values in the Range constructor
Rename Range to Region

range = Range(...) is too tempting and we probably don't want to
redefine keywords.
Update the docs
Merge pull request #336 from ufo-kit/improve-scan

Improve scan