Enforce parent blocks being empty in octree

[normanb]

When creating inner blocks the code enforces that the parent blocks entries are ignored (in octree.select for example) however the actual entries array of the parent block is still populated. Probably not an issue when using just a few objects but it is when using thousands.

The code comment says the parent block will be emptied but it isn't.

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[sebavan]

LGTM, nice catch, adding @deltakosh for final validation :-)

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