Cleanup

[nlohmann]

This PR performs some housekeeping after the latest merges:

• fix compiler and sanitizer warnings
• unify CMake build directory names for maintainer tasks
• add CMake message when implicit conversions are switched off
• update documentation
• extend gitignore file

[coveralls]

Coverage remained the same at 100.0% when pulling 3c7dc63 on cleanup into 43e07bb on develop.