this is an API change. Before it would fail and now it returns all conditions.

thinking about it we could use a regexp type matching as for find_selection.
So keep failing if None and more than one dataset is present and allow
things like:

all_evoked = read_evoked(..., condition='*')
auditory_evoked = read_evoked(..., condition='*auditory*')
visual_evoked = read_evoked(..., condition='*visual*')

what do people think about this?

we keep if condition is int or str that it returns only one evoked and not a list