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, in which each node of the ONP-tree data structure corresponds to a data container that stores a portion of the multi-level, multi-dimension data set, and in which each of the data containers corresponding to a direct ancestor node of a higher-level ONP node contains a projection of the data stored in the higher-level ONP-tree node; wherein the ONP-tree data structure comprises: meta-data; and a serialized and compressed ONP tree, the serialized and compressed ONP tree comprising a number of serialized and compressed hash objects within the ONP-tree data structure.