/// Returns the time on the line where a point is closest to the given point.
NumericT time_for_closest_point (const VectorT & p3) const {
auto p1 = _point;
auto p2 = _point + _direction;
auto d = _direction.length_squared();
NumericT t = 0;
for (dimension i = 0; i < D; ++i)
t += (p3[i] - p1[i]) * (p2[i] - p1[i]);
return t / d;
}
