Switch to new specification (N+1) and improve testing coverage

[frheault]

This is a roadmap for what is coming after a meeting with @anibalsolon.
To help development I will generate more exotic .TRX (un/compressed, small/large, with single/overlapping/no points, mismatched dpv&dps, etc).

• Test examples/load_trx.cpp
• Run benchmark on V1 of Figshare
• Change the code to support the N+1 offsets change both at load & save
• Run benchmark on V2 of Figshare
• Explore which Unit Tests could be useful (same as trx-python?)
• Deploy C++ Unit Tests on GitHub
• Autorun benchmark on GitHub (maybe using a command in PR?)
• Add a bit of detail to the README.md

@arokem had links in #1 for C++ and CI