Ensmallen 2.21 released - C++ library for numerical optimisation

Ensmallen 2.21.1 has bug fixes and feature enhancements.
Download: https://ensmallen.org/
Ensmallen is a C++ numerical optimisation library (L-BFGS, SGD, CMA-ES, etc)
Changes since version 2.20:
- fix numerical precision issues for small-gradient L-BFGS scaling factor computations
- fix return types of callbacks
- clarify return values for various callback types
- cleanup for printing optimization reports
- ensure the tests are built with optimisation enabled