Pass points by value

[Matistjati]

In geometry, it's mixed whether points are passed as P&, const P& or P. We should probably be consistent. Personally, I prefer just P. We should never have P&: then you can't do reasonable things such as:

auto eval = [&](double p) {
    return segDist(a, b, c + delta * p);
};

[Matistjati]

We probably shouldn't merge this yet. I haven't tested these changes, and there might be more changes to make. But I want to get the discussion going @simonlindholm

[simonlindholm]

Sounds like a good idea to me.