fmmlib: L2QBXL does unnecessary work
The L2QBXL implementation in the fmmlib backend iterates over all QBX centers, but it should ideally just iterate over all centers that have targets. (This is true of the M2QBXL implementation.)
The L2QBXL implementation in the fmmlib backend iterates over all QBX centers, but it should ideally just iterate over all centers that have targets. (This is true of the M2QBXL implementation.)