Fast Merging of RNTuple Data Sets

The RNTuple interface is an experimental I/O subsystem for the ROOT analysis framework. Preliminary benchmarks have demonstrated significant performance and space improvements over the legacy TTree system. RNTuple files are split into storage and metadata. Using metadata offset information, RNTuple files can be merged together without a costly storage decompression step. This project will implement a fast merge function to combine RNTuple data sets. The hadd command-line tool will be updated to take advantage of the new RNTuple merge features.