YAML as a drop-in replacement for libconfig

This commit introduces a substitute module library for setting up softmodems using YAML instead of libaconfig. The softmodems should work the same with both types of files. Extra care was put to make sure the behavior of both modules is the same, except for writing/modifying the config.

Example yaml config files were added for nrUE and gNB.

This merge request introduces third party library yaml-cpp (https://github.com/jbeder/yaml-cpp) under MIT license.