Nodes of the bspt. It is a binary tree so nodes contain two children,
called left and right.
Nodes split along one dimension. The instance variable dim holds
the dimension along which this node is split.
Each child holds the minimum and maximum values for its subtree
when split along the specified dim.
The current implementation allows for the case where the maximum
left value is == the minimum right value. This can happen when
the tree is filled with coordinate values that contain the same
value along one dimension ... as with very regular crystals
The tree is not kept balanced.