ENH: stats: return namedtuple for describe output

Addresses #3665. 
I just started to try out named tuples for the first time.
I didn't realize this defines a class. I think it's clearer in this case to keep it outside the function, there might also be some cases where it's actually used by several functions.

As bonus, I would prefer standard pep-8 classnames with capitalized/camelcase names, so uniformed developers like I don't get confused about classes versus instances.
Plus, I would use the same class name in the namedtuple definition and the assignment.

(Assuming I understand enough of namedtuples.)