@brief check the existence of an element in a JSON object @sa https://json.nlohmann.me/api/basic_json/contains/

can't use conversion operator because of ambiguity

@brief returns the number of occurrences of a key in a JSON object @sa https://json.nlohmann.me/api/basic_json/count/

! @brief check whether a string is UTF-8 encoded The function checks each byte of a string whether it is UTF-8 encoded. The result of the