Create per-level lists of centers

As a follow-up to !91 (merged), looping over centers for each level in M2QBXL to see if they're associated with a target box on that level seems likely to the benefits of !91 (merged). We should build per-level lists of centers whose target boxes are on that level, in order to be able to find those centers more quickly and with better load balance.

cc @haogao2