1. A data management system comprising: a computer system that includes one or more processors, one or more electronic memories, and a memory-mapped file facility that electronically stores data; an ordered, nested, partition-tree (“ONP-tree”) data structure stored as a memory-mapped file by the memory-mapped file facility; and data management system control routines, executed by the one or more processors, which retrieve a particular projection of a multi-level, multi-dimension data set stored within the ONP-tree data structure that contains the multi-level, multi-dimension data set.