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Multi-Objects: MBALL_OT_select_similar

Compared to previous implementation, the following has been changed:
* Threshold: is now an absolute value. This allows a comparison with e.g. radii
that are much larger than selected radius. This is also consistent with
`CURVE_OT_select_similar`

* Radius in world space is the average of the radius scaled in x, y and z
directions
* Since MetaBalls are symmetrical, rotation is only considered from 0 to ?/2.
So for example rotations of 90� and -90� are considered equal.
This is also consistent with the way `CURVE_OT_select_similar` works.

Fix/changes from committer (Dalai Felinto):
* Drawing not updating after changes. (see original patch for details).

Reviewers: dfelinto
Differential Revision: https://developer.blender.org/D3895