[Alpha_complex] new do_sqrt argument

[mglisse]

The filtration values we use in the alpha complex are squared radii. Most applications are more interested in a (non-squared) radius. We use squares because if we add weights, taking the square root becomes less meaningful, in particular if the value becomes negative. But I think we should make it as easy as possible for people to get the square root they expect, and a parameter seems to be the easiest, passing do_sqrt=True if we want the square root. The name do_sqrt is just a placeholder for the report, better names are welcome.