[mock_uss] Fix clustering

[the-glu]

The randomize function of the mock_uss's clustering is wrong and can generate clusters outside the list of points.

Keeping it in one axes, if min = 0, max = 4 and points = [1, 3], this mean

u_min = 1
u_max = 3
x_offset = random.uniform(-3, 1)

Should we chose -3 as value

x_min = -3 x_max = 1

Meaning our clustering window will have one point outside of the cluster.

This PR fixes it with another calculation with explanations, see the code.

This PR also fix extend that was wrongly used self instead of new cluster, and do a copy of points to avoid having the same list between cluster's copies.