Building the cell search tree was dominated by one small std::vector<int>
allocation per leaf, and a tree with one cell per leaf becomes very deep for
large grids.
- BoundingBoxTree: Store leaf ids in flat arrays, and move the input data
instead of copying it. The tree shape is unchanged. Queries no longer copy
the ids of each leaf.
- RigMainGrid: Always group at least 8 cells along K in each leaf.
findIntersectingCells() removes cells not intersecting the query bounding
box, so the result is the same set of cells as for a tree with one cell per
leaf. This also applies to very large grids, which previously got the
cells of the whole leaf.
- Include cells of nested LGRs and LGR cells in invalid main grid cells in
the grouped tree, as the previous tree with one cell per leaf did.
