Version 2.1.0 tagged

Anyone has any objection for me to tag a 2.1.0 release? If not, I will do so on Monday the 18/05.

A whole bunch of changes and fixes. Two remarkable ones which require an increased minor-version:

• improved CMake installation and nlohmann-json-detection
• an error is thrown at the end of set_root_schema if undefined references are still present (#97)

Thanks to all contributors for adding features and fixing bugs.