Clump::updateProperties discretization not defined robustly

Most likely the reason for performance drop reported in [1]

dx in the voxel decomposition is minRad/discretization. The problem is when a sphere of radius 1 is member of a clump which aabb has the size 1000 (for instance). Using the minimal discretization=1 gives a number of voxel = 1e9 in such case and it is no possible to go below this number. I think discretization should refer to aabb size, not member size. I have this fixed locally, let me know if you agree with the fix.

[1] https://answers.launchpad.net/yade/+question/268954

Original bug report by Bruno Chareyre on 2015-07-15 : https://bugs.launchpad.net/yade/+bug/1474956